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

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]

Add method complexity check to CI
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !1783

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

Change "recent" scopes to sort by "id"

These scopes can just sort by the "id" column in descending order to
achieve the same result. An added benefit is being able to perform a
backwards index scan (depending on the rest of the final query) instead
of having to actually sort data.

See merge request !1788

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