Add a system note and update relevant merge requests when a branch is deleted or re-added

If a branch is deleted with an open merge request, amended offline, and then pushed again,
GitLab doesn't bother to update the merge request even though the last commit ID and/or
code may have changed. This MR ensures that each push will update any relevant merge
requests and adds a system note if this happens as well.

The new messages look like:

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/6581eea069cd8e485b7fa4187ed4c043/image.png)

Closes #2926 

See merge request !1601

If a branch is deleted with an open merge request, amended offline, and then pushed again,
GitLab doesn't bother to update the merge request even though the last commit ID and/or
code may have changed. This MR ensures that each push will update any relevant merge
requests and adds a system note if this happens as well.

Closes #2926

Changed loose to lose

Implement when syntax in .gitlab-ci.yml

See merge request !1606

[ci skip]

Also adds a preamble note that a working 8.0 installation (i.e.,
gitlab-git-http-server, updated Nginx/Apache configs) is required before
proceeding.

[ci skip]

[ci skip]

Sentences end in periods.

[ci skip]

See merge request !1604

Prepare 8.1 documentation

[ci skip]

See merge request !1612

Update redcarpet gem. Fixes gem memory leak

Fixes #2773 

See merge request !1607

Build View UI fixes



See merge request !1608

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

Markdown typography CSS fixes

The `!important` flags recently added to the `md-typography`
mixin's `pre` elements were overriding the background and
foreground colors for syntax highlighting.

| Before | After |
|:------:|:-----:|
| ![Screen_Shot_2015-09-25_at_3.32.32_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/802163bd9f46864ac89e7be165951f07/Screen_Shot_2015-09-25_at_3.32.32_PM.png) | ![Screen_Shot_2015-09-25_at_3.33.30_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/6be72036653378405320085324dff5ad/Screen_Shot_2015-09-25_at_3.33.30_PM.png) |

See merge request !1442

Improve invalidation of stored service password if the endpoint URL is changed (V2)

New version of !1583, using the same failproof logic but this time mirroring the name and behaviour of the change-tracking methods of ActiveModel::Dirty in order to make it clearer and more natural.

Added more tests to clarify the expected behaviour.

This is an alternative to !1594

/cc @vsizov @rspeicher

See merge request !1600

Improve User.by_login performance

This greatly speeds up the performance of `User.by_login`. I adopted some changes from @haynes in this patch, the credits go to him for coming up with those originally.

Fixes #2341

See merge request !1545

Let gitlab-git-http-server handle archive downloads

This change relies on changes in gitlab_git and gitlab-git-http-server.

fixes #2429

See merge request !1548