Add option to mirror an upstream repository.
Closes internal https://dev.gitlab.org/gitlab/gitlab-ee/issues/279
Depends on gitlab-org/gitlab-shell!29
To do:
- [x] Decide on what user should be the author of the activity feed events. The initial project creator perhaps? That makes sense for personal projects, but less so for group projects, where it's even possible that the creator has since left the team.
- [x] Write specs!
- [x] Write documentation
- [x] Port back relevant commits to CE: gitlab-org/gitlab-ce!1822
----
## Mirror status on project homepage

## Settings navigation item

## Mirroring settings

## Activity feed with new "pushes" (mirror updates)

I have yet to decide on what user should be the author of these events. The initial project creator perhaps? That makes sense for personal projects, but less so for group projects, where it's even possible that the creator has since left the team.
## Warning that branch has diverged from upstream

## Commits heading after failed update

Link send user to the mirroring settings if they are a project admin.
## Mirroring settings after failed update

## New project page

## Import form after failed import
(Screenshot outdated)

Shown when the initial import of a project fails, not when an update of an existing project fails.
cc @dzaporozhets @sytses @JobV
See merge request !51
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.