Allow user to customize default Dashboard page

Renames the "Design" profile page to "Preferences" and adds a field to customize the default Dashboard page:

> ![Screen_Shot_2015-06-11_at_11.12.53_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/b5282a3be7861d1148528c6bc9e7a0e0/Screen_Shot_2015-06-11_at_11.12.53_PM.png)

See merge request !778

Remove visibility icon from projects list on dashboard and group page

## Why?

Because

* visibility level show on project page anyway
* it does not have much value. Main purpose of project list on dashboard is navigation. Avatar + project name helps this goal. Visibility icon have no effect here
* it makes UI worse because it brings separation between project avatar and name
* without visibility icon we have more space for project name. Especially important for laptops

## Screenshot

![Screenshot_2015-06-12_18.17.20](https://gitlab.com/gitlab-org/gitlab-ce/uploads/0e572388e9a337c5a535813911d08c89/Screenshot_2015-06-12_18.17.20.png)

P.S. Merge when 7-12-stable is created

See merge request !806

Remove button to all projects on Trending Projects page.

Fixes #2001.

See merge request !1860

Fix post-receive errors on a push when an external issue tracker is configured

### What does this MR do?

This MR improves upon !766, fixing errors upon a git push that occur only when an external issue tracker (not JIRA) is used. This MR takes into account that external issue trackers, such as JIRA, may close issues. Disabling the processing of post-receive commits when an external issue tracker is configured seems like the wrong behavior.

### Why was this MR needed?

When a user adds an issue reference, the refactoring in 8f8a8ab3 and a6defd15 caused `project.get_issue` to be called, causing `ExternalIssue` to be returned when an external issue tracker was configured. This object does not have a `close` method, as needed in `CloseService`. Nor does it make sense to associate a `SystemNote` with this object.

GitLab EE uses a [special case for JIRA](https://gitlab.com/gitlab-org/gitlab-ee/blob/master/app/services/git_push_service.rb#L87). I would recommend moving this logic into `CloseService` too.

### What are the relevant issue numbers?

* Closes #1700
* Closes #1720

See merge request !804

Update Irker home page

[ci skip]

Closes #1713

See merge request !812

[ci skip]

Closes #1713

Fixes #2001.

Update noteable after a new note is added

Update Redcarpet to 3.3.0

Fixes #1432

See merge request !809

Prepare specs for RSpec 3

See merge request !811

Also converts doc/README.md to Unix line endings

This controller is now the target for `root_url`. It sub-classes
DashboardController so we can render the old default without a redirect
if the user hasn't customized their dashboard location.

It was only used for the appearance live updating, which is now handled
by Profiles::Preferences#update

Consolidates the helpers related to user preferences.

Renames `app_theme` to `user_application_theme` to better explain what
it is.

Now we can simply loop through all themes, among other things.

This also removes the `dark_theme` / `light_theme` classes and the
`theme_type` helper, since they weren't used anywhere.

Since they're all defined by a mixin, it didn't provide any benefit to
have them in separate files.

This also adds variables defining the basic color of each theme so we
can re-use them in the previews.