Fix error 500 when no HEAD is available

Steps to reproduce:

1. Create a project with a README
2. In the actual remote, type: `git symbolic-ref HEAD refs/heads/nowhere`
3. Check that HEAD is gone via `git ls-remote .`
4. Go to the projects page and see the Error 500

Error:

```
NoMethodError - undefined method `sha' for nil:NilClass:
  app/helpers/projects_helper.rb:281:in `readme_cache_key'
  app/views/projects/_readme.html.haml:10:in `_app_views_projects__readme_html_haml___2036282917939462960_70154565285700'
  actionview (4.1.11) lib/action_view/template.rb:145:in `block in render'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `block in instrument'
  activesupport (4.1.11) lib/active_support/notifications/instrumenter.rb:20:in `instrument'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `instrument'
  actionview (4.1.11) lib/action_view/template.rb:339:in `instrument'
  actionview (4.1.11) lib/action_view/template.rb:143:in `render'
  rack-mini-profiler (0.9.0) lib/mini_profiler/profiling_methods.rb:108:in `block in profile_method'
  actionview (4.1.11) lib/action_view/renderer/partial_renderer.rb:306:in `render_partial'
  actionview (4.1.11) lib/action_view/renderer/partial_renderer.rb:279:in `block in render'
  actionview (4.1.11) lib/action_view/renderer/abstract_renderer.rb:38:in `block in instrument'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `block in instrument'
  activesupport (4.1.11) lib/active_support/notifications/instrumenter.rb:20:in `instrument'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `instrument'
  actionview (4.1.11) lib/action_view/renderer/abstract_renderer.rb:38:in `instrument'
  actionview (4.1.11) lib/action_view/renderer/partial_renderer.rb:278:in `render'
  actionview (4.1.11) lib/action_view/renderer/renderer.rb:47:in `render_partial'
  actionview (4.1.11) lib/action_view/helpers/rendering_helper.rb:35:in `render'
  haml (4.0.5) lib/haml/helpers/action_view_mods.rb:10:in `block in render_with_haml'
  haml (4.0.5) lib/haml/helpers.rb:89:in `non_haml'
  haml (4.0.5) lib/haml/helpers/action_view_mods.rb:10:in `render_with_haml'
  app/views/projects/show.html.haml:47:in `_app_views_projects_show_html_haml__1458303859692972698_70154948164180'
  actionview (4.1.11) lib/action_view/template.rb:145:in `block in render'
  activesupport (4.1.11) lib/active_support/notifications.rb:159:in `block in instrument'
```

Closes https://github.com/gitlabhq/gitlabhq/issues/9484, but there may be some other issue there.

See merge request !1041

Add admin_merge_request ability to Developer level and up

This was necessary because this permission is checked dynamically by
`app/views/shared/issuable/_context` when on a Merge Request.

Closes #2058

See merge request !1037

This was necessary because this permission is checked dynamically by
`app/views/shared/issuable/_context` when on a Merge Request.

Closes #2058

Steps to reproduce:
1. Create a project with a README
2. In the actual remote, type: `git symbolic-ref HEAD refs/heads/nowhere`
3. Check that HEAD is gone via `git ls-remote .`
4. Go to the projects page and see the Error 500

Closes https://github.com/gitlabhq/gitlabhq/issues/9484

Allow custom label to be set for authentication providers.

Addresses internal issue https://dev.gitlab.org/gitlab/gitlabhq/issues/2341

Beside the above, I've:
- Refactored `OauthHelper` to have clearer method names and behaviour
- Moved some of `OauthHelper` behaviour to `Gitlab::OAuth::Provider`
- Renamed `OauthHelper` to `AuthHelper` since LDAP, SAML, Kerberos aren't OAuth
- Updated the icons for GitHub and GitLab

In the examples below, "OurAuth" is a SAML provider with a custom label.

![Screen_Shot_2015-07-02_at_16.29.52](https://gitlab.com/gitlab-org/gitlab-ce/uploads/7d425bde69dc34e1667ebd5375d0266d/Screen_Shot_2015-07-02_at_16.29.52.png)

![Screen_Shot_2015-07-02_at_16.31.40](https://gitlab.com/gitlab-org/gitlab-ce/uploads/cbb273321ecdf4cab3d3ef0dc35553e7/Screen_Shot_2015-07-02_at_16.31.40.png)

![Screen_Shot_2015-07-02_at_16.32.39](https://gitlab.com/gitlab-org/gitlab-ce/uploads/d8dd6e1d0dc45a788e869cdcdc99e178/Screen_Shot_2015-07-02_at_16.32.39.png)

![Screen_Shot_2015-07-02_at_16.33.18](https://gitlab.com/gitlab-org/gitlab-ce/uploads/7dbfe8b0ae229c32a08d6c7442976d83/Screen_Shot_2015-07-02_at_16.33.18.png)


See merge request !927

Fix and style the "Changes suppressed" link.

At some point the "Changes suppressed. Click to show" link stopped behaving as expected, so this fixes it.

I also thought it looked bad:

![Screen_Shot_2015-07-23_at_4.06.49_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/0a1552e4edb659f065bd202b93bc61ef/Screen_Shot_2015-07-23_at_4.06.49_PM.png)

So I gave it some room to breathe:

![Screen_Shot_2015-07-23_at_4.00.25_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/abc0704e597ed010cade81782ac9099e/Screen_Shot_2015-07-23_at_4.00.25_PM.png)


See merge request !1038

Make username parameter for Users#show case-insensitive

Closes #2012

See merge request !996

Update changelog in master



See merge request !1036

Changes for gmail inbox actions

Related to the ongoing efforts to get Gmail inbox actions for GitLab.com in #1517

Not using `mdash` would break the rendering of the html email by switching `Content-Transfer-Encoding` to `quoted-printable` for every email.

Removed the rake task and docs to prevent users from sending dummy emails which might cause application rejection.

See merge request !1024

Fix bug causing "Remove source-branch" option not to work for merge requests from the same project.

Fixes #2049.

See merge request !1025

Revert cache for events

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/1014#note_1661322

See merge request !1027

Revert cache issue

It reverts https://dev.gitlab.org/gitlab/gitlabhq/commit/40e31e1db31e6ba9a1cd6255afc2f09df61f4060

and fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/2048
and https://gitlab.com/gitlab-org/gitlab-ce/issues/2043

See merge request !1028

Fix link to 2fa help page.  Closes #2055

See merge request !1026

Check if session_expire_delay column exists before adding the column.

Fixes #2052 

Fixes #1985

See merge request !1029

revert caching of note text

This MR reverts the caching of note texts because what is rendered as a link in Markdown depends on the user's access to other projects and groups.

Until we find a solution that doesn't decrease security, let's not use the cache for this.

See discussion in gitlab-org/gitlab-ce!911 for more information.

/cc @DouweM @stanhu @vsizov

See merge request !1014

Speed up security feature specs

Before: `rspec spec/features/security/  0.12s user 0.04s system 0% cpu 3:38.00 total`

After: `rspec spec/features/security/  0.12s user 0.04s system 0% cpu 1:40.58 total`

The majority of the speed improvements is from two things:

1. Instead of using our standard `login_as` helper in the matchers, we take advantage
   of the `Warden::Test::Helpers` version of the method which bypasses the login form
   and logs the user in directly. We were essentially testing that filling out the login
   form works hundreds of times.
2. There were many tests that verified if a user of a certain access level
   (master, owner, reporter, guest) had access to a resource. Unfortunately we were
   creating every type of user for each test even though a test was only verifying one of
   them at a time. Now the tests only create the one user role they're testing.

See merge request !1023

Added Fork Project document to GitLab Basics



See merge request !1909

The user being created was no different than what `be_allowed_for` was
doing behind the scenes so we were essentially testing all user-level
accesses twice.

It was only used in one test, and the test was kind of redundant.