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**

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/9b3d405bf7b5f945e35bae3534c2b67b/image.png)

**New MR page**

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/3657a2a9efad6d10e8470637d1166bdb/image.png)

**System note**

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/cc8066f3d3bdf09c0cce27193210567d/image.png)

### 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!

See merge request !738