Add to_reference method to referable models

Now there is a single source of information for which attribute a model uses to be referenced, and its special character.

See merge request !641

Add support for manually entering 2FA details

> ![Screen_Shot_2015-05-22_at_6.39.26_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/27b6cc0b2f88e0ccec234dd8d20db957/Screen_Shot_2015-05-22_at_6.39.26_PM.png)

Related to #1679 though I'm not sure if it entirely fixes it, since I don't have access to a Windows Phone.

See merge request !694

Add a note when an Issue or Merge Request's title changes

> ![Screen_Shot_2015-05-26_at_9.40.19_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/f05c6a7b9f489ce66560cfe724184ec9/Screen_Shot_2015-05-26_at_9.40.19_PM.png)

Closes #1434

See merge request !717

[ci skip]

Allow HipChat API version to be blank and default to v2

### What does this MR do?

This MR fixes a regression introduced in v7.11 that requires a HipChat API version to be specified when it is supposed to be optional.

### Why was this MR needed?

The "optional" HipChat API version in 57c72455 passed in a blank `api_version` when nothing was specified, and the code was not tested. This would cause a 500 Error.

### What are the relevant issue numbers?

Closes #772

See merge request !718

ADD: Note on how to setup dns hostname resolution to installation manual

Hi,

I'm suggesting to add the following note to the installation instructions. It took me some time to figure out why the test failed and why I wasn't able to push. The error messages did not point into the right direction so this might help other people with the same problems.

I hope my english is good enough, if not, please feel free to rephrase anything.

Thank you in advance,

Holger Segnitz

See merge request !236

Closes #772

Fix "Asignee" typo in new issue/merge request emails

See merge request !676

We only want them shortened by the filter, which calls to_s

Except for Note, which still overrides it.

Snippet model was missing project association

Now there is a single source of information for which attribute a model
uses to be referenced, and its special character.

Prior, comparison would use the Ruby object's ID, which got out of sync
after a Spring fork and would result in erroneous test failures.

Now we just check that the compared object is a Commit and then compare
their underlying raw commit objects.

Change percent_complete rescue value from 100 to 0

The percent_complete method returns a value of 100 when a
ZeroDivisionError occurs. That seems like a very strange default for an
error case, and results in a bug when a milestone has no corresponding
issues (new, empty milestones show 100% completion). This commit changes
the rescue value to 0, and subsequently fixes #1656, which reported this
problem.

See merge request !714

The percent_complete method returns a value of 100 when a
ZeroDivisionError occurs. That seems like a very strange default for an
error case, and results in a bug when a milestone has no corresponding
issues (new, empty milestones show 100% completion). This commit changes
the rescue value to 0, and subsequently fixes #1656, which reported this
problem.

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

Validate wiki page creation

Fix Zen Mode not closing with ESC key

### What does this MR do?

This MR fixes the ESC key not leaving Zen Mode (fullscreen).

### Why was this MR needed?

This has been broken since GitLab v7.8. The `change` event for `zen-toggle-comment` was never fired. See:

http://stackoverflow.com/questions/19505011/checkbox-checked-with-prop-does-not-fire-events-attached-to-change?answertab=votes#tab-top

### What are the relevant issue numbers?

* Closes #1025
* Closes https://github.com/gitlabhq/gitlabhq/issues/9018

See merge request !710