Restricted visibility levels - bug fix and new feature

This allows admin users to override restricted visibility settings when creating and updating projects and snippets, and moves the restricted visibility configuration from gitlab.yml to the web UI.  See #1903.

## Move configuration location

I added a new section to the application settings page for restricted visibility levels.  Each level has a checkbox, styled with Bootstrap to look like a toggle button.  A checked box means that the level is restricted.  I added a glowing text shadow and changed the background color for checked buttons because the default styles made it hard to distinguish between checked and unchecked.  This image shows the new section with the "Public" box checked:

![restricted_visibility_settings](https://dev.gitlab.org/Okada/gitlabhq/uploads/629562e4313f89b795e81c3bb0f95893/restricted_visibility_settings.png)

## Allow admins to override

To allow admin users to override the restricted visibility levels, I had to remove the `visibility_level` validation from the `Project` class.  The model doesn't know about the `current_user`, which should determine whether the restrictions can be overridden.  We could use the creator in the validation, but that wouldn't work correctly for projects where a non-admin user is the creator and an admin tries to change the project to a restricted visibility level.

The `Project::UpdateService` and `Project::CreateService` classes already had code to determine whether the current user is allowed to use a given visibility level; now all visibility level validation is done in those classes.  Currently, when a non-admin tries to create or update a project using a restricted level, these classes silently set the visibility level to the global default (create) or the project's existing value (update).  I changed this behavior to be more like an Active Model validation, where using a restricted level causes the entire request to be rejected.

Project and personal snippets didn't have service classes, and restricted visibility levels weren't being enforced in the model or the controllers.  The UI disabled radio buttons for restricted levels, but that wouldn't be difficult to circumvent.  I created the `CreateSnippetService` and `UpdateSnippetService` classes to do the same restricted visibility check that the project classes do.  And since I was dealing with snippet visibility levels, I updated the API endpoints for project snippets to allow users to set and update the visibility level.

## TODO

* [x] Add more tests for restricted visibility functionality

cc @sytse @dzaporozhets

See merge request !1655

Fix typos

Fix minor typo in docs and CHANGELOG.

See merge request !394

Add documentation for HipChat integration

This was a source of confusion here:

https://github.com/gitlabhq/gitlabhq/issues/7373

See merge request !393

Auto config git if user forgot

If git is not configured properly, let gitlab:check try to fix it by executing the commands itself instead of outputing how the user could fix it.

If this fails the error is still displayed.

Edit;
Not yet added to the CHANGELOG as;
* Im not really sure this fits in the scope of a test
* No clue how to test this properly

See merge request !383

Use same layout and interactivity for project members as group members and make code more consistent.

It's probably easiest to review the commits one by one, but keep in mind that later commits sometimes touch the same place as earlier ones, so look at the complete diff to add comments.

The project members page and group members page now look and work the same, with an inline "Add members" form, member search and editing of access levels and deleting without page refresh.

Before:

![Screen_Shot_2015-03-13_at_16.50.39](https://dev.gitlab.org/gitlab/gitlabhq/uploads/2cd023c48de0a643ce04df965f39e1f5/Screen_Shot_2015-03-13_at_16.50.39.png)

After:

![Screen_Shot_2015-03-13_at_16.50.02](https://dev.gitlab.org/gitlab/gitlabhq/uploads/fdef480daf2859ed74f5641af1f337b3/Screen_Shot_2015-03-13_at_16.50.02.png)

See merge request !1695

Use pre-wrap for diff code in discussion view

This prevents merge request comments from being hidden in discussion view.  See #2017.

### Before

![diff_comment_before](https://dev.gitlab.org/Okada/gitlabhq/uploads/e6d24987b8d88f4e8a5883645d569b56/diff_comment_before.png)

### After

![diff_comment_after](https://dev.gitlab.org/Okada/gitlabhq/uploads/fc035f9fc44718dc8424d3f8b54f0953/diff_comment_after.png)

This change doesn't affect the "Changes" tab:

![diff_view_comment](https://dev.gitlab.org/Okada/gitlabhq/uploads/9975d4c3feac629d6a0d76b5a0ef43d8/diff_view_comment.png)

cc @sytse

See merge request !1700

Conflicts:
	db/schema.rb

Add Rspec tests for the new UpdateSnippetService and
CreateSnippetService classes.

Allow authors and admins to update the visibility level of personal and
project snippets.

Fix typo

Restyle import

Render import providers in one line and make it buttons

See merge request !1699

More flatly

* use sass variables for repeating colors
* use more of Flatly theme variables
* refactor buttons

See merge request !1698