Remove fork relationship from projects settings #2578

What does this MR do?

Adds the ability for project owners to remove the forked relationship to the source project via the Project > Settings screen. This was from an issue request #2578 (closed).

I worked on this MR with @DouweM.

Where should the reviewer start?

There's a new controller action projects#remove_fork and changes to the edit view to provide the new context. The logic was taken form the exiting API.

How should this be manually tested?

1. As a logged in user fork a project from another namespace
2. Visit the settings page for that newly fork project
3. You should be given the option to Remove forked relationship with a confirmation.
4. A flash message should appear when complete.

Screenshots