refactor login as to be impersonation with better login/logout

Modifies the existing "login as" feature to be called impersonation.

This also adds:

* Application keep track of who is impersonating the user so they can revert back to the original user without having to log out.
* Stores the user profile via `HTTP_REFERER` so you get redirected back to the person you have impersonated once you stop.

## Screenshots:

![](http://sindacio.us/i/2015-10-28_17-52-41.png)
![](http://sindacio.us/i/2015-10-28_17-53-08.png)

See merge request !1702

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

See merge request !1799

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

Expose builds feature

Expose builds feature in project settings (as feature).

Enable it by default for a new projects.

I deliberately named it builds instead of CI, because we actualy allow to run tests using infrastructure built-in GitLab. I'm free to change it.

![Screen_Shot_2015-11-09_at_16.42.21](/uploads/a8af0a56fc0498688c0428ff22252d9c/Screen_Shot_2015-11-09_at_16.42.21.png)

If we are ok, I'll add feature tests for it.

/cc @sytses @dzaporozhets 




See merge request !1767

Refactor complex methods

Make flog part of CI check which is not allowed to fail. I used high score (70) and refactored most complex method. In future releases we should lower acceptable score to something like 40..50

Part of #3444 

See merge request !1794

Bump Gitlab Workhorse



See merge request !1795

- Enable CI by default for all new projects

Add caching for ApplicationSetting, Ci::ApplicationSetting.

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

Add ignore white space option in merge request diff

fix this issue(https://gitlab.com/gitlab-org/gitlab-ce/issues/1393).

Add ignore whitespace optoin to Commits Compare view

Add missing "omniauth" prefix to option in docs

This MR is specifically a fix for #3319. I wanted to make my first contribution to GitLab, so I picked a simple one!

This changes the omniauth documentation for the `block_auto_created_users` option to `omniauth_block_auto_created_users`.

Thanks

---

closes #3319 

See merge request !1734

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

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

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

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

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

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

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

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

Use a relative link instead of full URL with New Issue button to be consistent

Relates to #3095

See merge request !1631

It is confusing to have two administrator documentation sections.



See merge request !52

Changes block_auto_created_users to omniauth_block_auto_created_users,
otherwise the option is ignored. Fixes #3319.

Avoid render edit_form in each notes.

Remove deprecated dumped yaml file generated from previous job definitions



See merge request !1777

Fix "Other Builds" links in Projects Build page

Found a minor issue where all links in the "Other Builds" section of Project Builds was linking to the same build #. This should fix it.

See merge request !1785

Allow to define cache in `.gitlab-ci.yml`

This extends `.gitlab-ci.yml` syntax to allow specifying caching files and directories between builds, making it easy to preserve ex. gems.

```
cache:
  paths:
  - .bundle
  - vendor/

before_script:
- bundle install --path vendor/

rspec:
  script:
  - bundle exec rspec
```

This is based on Build Artifacts changes.

/cc@dzaporozhets 

See merge request !1786

ApplicationSetting.current was called in every pages, cache it and expires it after it updated.

This changes will avoid a SQL query in every pages (~0.3 - 0.5ms).

```SQL
SELECT  "application_settings".* FROM "application_settings"   ORDER BY "application_settings"."id" DESC LIMIT 1
```

Use RJS to render edit note feature.

Before:

```
Rendered projects/notes/_note.html.haml (27.9ms)
Rendered projects/_zen.html.haml (0.3ms)
Rendered projects/notes/_hints.html.haml (0.7ms)
Rendered projects/_md_preview.html.haml (3.9ms)
Rendered projects/notes/_edit_form.html.haml (6.9ms)
Rendered projects/notes/_note.html.haml (17.7ms)
Rendered projects/_zen.html.haml (0.3ms)
Rendered projects/notes/_hints.html.haml (0.6ms)
Rendered projects/_md_preview.html.haml (3.4ms)
Rendered projects/notes/_edit_form.html.haml (7.0ms)
```

After:

```
Rendered projects/notes/_note.html.haml (13.8ms)
Rendered projects/notes/_note.html.haml (7.1ms)
Rendered projects/notes/_note.html.haml (9.5ms)
Rendered projects/notes/_note.html.haml (8.5ms)
```

This change reduce at least 6ms * N ('N' - number of notes).

Add flay: tool to find duplicate code
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !1789

[ci skip]