Support editing target branch of merge request

Stan Hurequested to merge
stanhu/gitlab-ce:support-edit-target-branch-in-mr into master

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

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New MR page

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System note

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What are the relevant issue numbers?

Merge request reports