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.

Use project repository expire_cache to clear cache in test

Clearing the entire Redis cache in 643557da for the `branches_spec.rb` may be causing issues. Just clear what we need.

See merge request !1022

the repository expire_cache method

Remove 7.14 entry that was pushed to 7.13

See merge request !1020

Fix 404 error in files view after deleting the last file in a repository

Here's the logic:

1. In `TreeController`, `require_non_empty_project` will prevent `show` from being called if the project is  empty. That means all calls to `show` will be guaranteed to have at least 1 commit.
2. If the ref name is not valid, then return a 404. This ensures that at least the controller is looking at a valid branch/tag/SHA ID.
3. This leaves a number of cases:

```
3a. Valid ref, valid directory
3b. Valid ref, valid filename
3c. Valid ref, invalid path
3d. Valid ref, no files
```

Case 3a: The tree will not be `empty?` and will pass through the whole function.

Case 3b: The tree will be `empty?` but the `blob_at` will resolve properly and trigger a redirect to the file.

Case 3c: In this case, a path is given. Return 404 if it cannot be resolved neither as a tree nor a blob.

Case 3d: In this case, no path is given. If the tree is empty, this means it's an empty branch and just fall through.

Example broken branch: https://gitlab.com/gitlab-org/gitlab-test/tree/empty-branch

Closes #1362

See merge request !1010

Fix full screen mode for snippet comments

ZenMode is now initialized for snippet pages.

Fixes #2036.

See merge request !1016

Closes #1362

ZenMode is now initialized for snippet pages.

Fixes #2036.

You need to store your DB encryption key

You can break two-factor authentication for existing users when
restoring a backup if you lost your DB encryption key.

See merge request !1015