Add ability to remove the fork relationship from project settings


Replaces !1579.
Fixes#2578.
See merge request !1636
This was previously possible through the API but can now be done
through the project#edit settings screen if the current user is
the owner of the project.
Update changelog
Notifications button was unavailable if user wasn't member of the
project, even if protected project is available via group privileges.
Showing disabled button with explanation tool-tip is less confusing.
This closes#2846.
- Split up SnippetsController into separate dashboard and explore sections.
- Use consistent page titles, header titles and sidebars between dashboard and explore sections when signed in or not.
Ths commit does next:
* When we remove project we move repository to path+deleted.git
* Then we schedule removal of path+deleted with sidekiq
* If repository move failed we abort project removal
This should help us with NFS issue when project get removed but
repository stayed. The full explanation of problem is below:
* rm -rf project.git
* rm -rf removes project.git/objects/foo
* NFS server renames foo to foo.nfsXXXX because some NFS client (think
* Unicorn) still has the file open
* rm -rf exits, but project.git/objects/foo.nfsXXX still exists
* Unicorn closes the file, the NFS client closes the file (foo), and the
* NFS server removes foo.nfsXXX
* the directory project.git/objects/ still exists => problem
So now we move repository and even if repository removal failed
Repository directory is moved so no bugs with project removed but
repository directory taken. User still able to create new project with
same name. From administrator perspective you can easily find stalled
repositories by searching `*+deleted.git`
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>