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

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

Fixes #1432

Ensure `session_expire_delay` field exists before accessing it

Closes #1798

See merge request !808

Closes #1798

Add info about regex anchors to shell command docs.

Addresses internal issue https://dev.gitlab.org/gitlab/gitlab-ee/issues/263.

See merge request !805

Closes #1700
Closes #1720

Add session expiration delay configuration through UI application

Setting is accessible by the administrator through the UI and defaults to 1 week (the current setting)

Answers the following suggestions:

  * http://feedback.gitlab.com/forums/176466-general/suggestions/6210719-make-session-length-configurable
  * http://feedback.gitlab.com/forums/176466-general/suggestions/6730512-automatic-logout-after-a-time-being-idle

See merge request !774

Add 7.12 update guides

See merge request !1858

Fix typo on account page.

@douwe Thanks for reviewing this small fix.

See merge request !1854

Update install docs for 7.12

See merge request !1857

Even though the 7-12-stable branch does not exist yet.

Git 2.1.2 does not protect against pushes trying the '.Git/config'
attack (CVE-2014-9390). Going to 2.4.3 is perhaps a big jump but why not
use the latest? (Famous last words.)

Remove text since rss is now to prominent.

See merge request !1855

Fix link to howto_rc1.md

The relative link that was there before breaks when you copy-paste the
issue template into an actual issue.

See merge request !1856

The relative link that was there before breaks when you copy-paste the
issue template into an actual issue.

Prepare feature steps for RSpec 3

See merge request !803

There's a naming conflict between Capybara and rspec-matchers which both
define the `all` method.

See https://github.com/jnicklas/capybara/issues/1396

Add a note that changing example configuration files requires changing omnibus-gitlab.

Keeping track of changes in these files is not easy and they usually require change in omnibus-gitlab.

Added a note so everyone is aware of this.

Similar is added to gitlab-shell and gitlab-ci.

Discussed in gitlab-org/omnibus-gitlab#605

See merge request !800

Refactor accept merge request widget

* make each case a simple view
* separate  merge request JS from merge request accept widget JS
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !799

Fix graph width on different screen sizes
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !802

Make commit calendar size depend on window size
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !801

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>