Fix OAuth provider bug where GitLab would not go return to the redirect_uri after sign-in

### What does this MR do?

This MR fixes a bug when GitLab is used as an OAuth provider (e.g. to GitLab CI) where the user is not redirected back to the original URI after a successful sign-in. This MR tweaks the Doorkeeper configuration to set the `user_return_to` Devise session variable, as mentioned in this [Stack Overflow](http://stackoverflow.com/questions/14504739/how-to-use-both-devise-and-doorkeeper-gems) post.

### Why was this MR needed?

If you attempt to "Login via GitLab" from GitLab CI and aren't logged out completely, GitLab asks for your username and password. After you login, you get directed to the GitLab dashboard instead of back to GitLab CI. It's easy to reproduce:

1. Set up a GitLab CI and GitLab instance (e.g. ci.gitlab.com, gitlab.com).
2. Start an Incognito Window so that you are logged out of GitLab CI and GitLab.
3. Go to the GitLab CI instance. Click the "Login with GitLab".
4. Enter in your credentials.
5. Watch your browser login to GitLab and not return to GitLab CI.

### What are the relevant issue numbers?

Closes #1612

See merge request !998

Closes #1612

Show the first tab automatically on MergeRequests#new

Closes #2013

See merge request !997

Closes #2013

Only load byebug on MRI

This Gem relies on MRI specific debugging APIs. This at least allows
installation on Rubinius/JRuby.

Fix file upload dialog for comment editing

This fixes the file upload dialog which currently works for new comments
but not for comment editing. The cause for this is a missing DOM class
which causes traversal to the dropzone to fail.

This is fixed by adding the expected `gfm-form` class to the edit form.
It has to be added dynamically since adding it to the form template
would break the form's layout and comment saving.

The following screenshot illustrates the issue:
![gitlab-dropzone-upload-dialog-bug](https://gitlab.com/gitlab-org/gitlab-ce/uploads/3a9885c8ba7c07338148ec9e605e5771/gitlab-dropzone-upload-dialog-bug.png)

See merge request !956

Add test case for clicking line number anchor icon

Follow-up to !988

See merge request !992

Fix bug where clicking on the link icon on a file would result in "#Lundefined" in the URL

Closes #2002

See merge request !988

This fixes the file upload dialog which currently works for new comments
but not for comment editing. The cause for this is a missing DOM class
which causes traversal to the dropzone to fail.

This is fixed by adding the expected `gfm-form` class to the edit form.
It has to be added dynamically since adding it to the form template
would break the form's layout and comment saving.

Add support for destroying project milestones

### What does this MR do?

This MR adds a "Remove" button to the project milestones page and the milestone page itself.

### Why was this MR needed?

Because lots of people talked about needing to clean their toilets. :)

### What are the relevant issue numbers?

Closes https://github.com/gitlabhq/gitlabhq/issues/1504

### Screenshots

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/ef8c7a3ea1db7b37cccae3869ac4de0a/image.png)

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/33eb01c7bc30fb235de96db5efb8746d/image.png)

See merge request !980

Validate format of project_url and token for GitLab CI service.

If `project_url` and `token` for are invalid, [service_hook creation](https://gitlab.com/gitlab-org/gitlab-ce/blob/7-13-stable/app/models/project_services/gitlab_ci_service.rb#L30-34) will silently fail due to validation of URL in `WebHook`.

Given that token is a sequence of numbers and letters for GitLab CI making sure that there are no unexpected characters should be enough to prevent service_hook being nil. 

Fixes #1997 

See merge request !987

Avoid copy of strings in memory for parsing git grep result
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

See merge request !967

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

Advanced cache

Fixes #1993

* Build missing cache values in background job after each push
* Store commit_count in project table. Refresh in background job
* moved repository size calculation in background job


## Advantages

Every time push is triggered - we build cache for project even without user visiting project page. 
That means first visit is as fast as others now. This is especially good for active projects where people have some requests fast because of cache and some slow - because cache was build in their request. 

Between cache expired and cache built we we had gap when Linux repo can give 502 error because calculation commits count takes 30 seconds or even more. 
Using value from database fix this problem. Before cache is updated you see old value from database. After - you see new one. 

Basically this merge request is super win to GitLab. We don't do some heavy operations in user request but instead in background job. 

## Temporary problem

After this migration all projects will have `0` commits in database. It fill be replaced with real value on next push. I did not add recalculation to migration because it will last forever on big instances.  Can be fixed by adding rake task which will go in background on live instance without downtime

See merge request !986

The release manager owns the stable branches

I think this helps the release manager because it means they don't
have to worry about others pushing commits at the last minute.

See merge request !1907

Expire Rails cache entries after two weeks

This should help prevent endless Redis growth.

See merge request !1903

Closes #2002

I think this helps the release manager because it means they don't
have to worry about others pushing commits at the last minute.

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

Closes https://github.com/gitlabhq/gitlabhq/issues/1504

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>

Added note about chef

Added note about chef being required to generate GitLab config. because of tweet https://twitter.com/grahamgilbert/status/621015318442459136

See merge request !982

Added new doc about creating a new branch

Added new document about creating a branch and fixed bullets in all the other documents. Also fixed "git push REMOTE BRANCH".

See merge request !1894

Document should start from h1 title. Also put badges on top for easier tracking

Looks much better with new project page design. 

See merge request !983