Support editing target branch of merge request

What does this MR do?

This MR makes it possible to edit the target branch of a merge request and adds a system note when this happens.

Why was this MR needed?

Because lots of people requested this feature. :)

Screenshots

Edit MR page

New MR page

System note

What are the relevant issue numbers?

• Closes https://github.com/gitlabhq/gitlabhq/issues/7105
• See: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/130!