Be able to create a copy of merge request when editing and selecting different target branch

Issue idea came from: http://blog.ffwll.ch/2017/08/github-why-cant-host-the-kernel.html

Usecase

Currently you can switch the merge request to target a different branch. However you may want to merge those changes into multiple branches.

Proposal

Offer an (checkbox) option to "create a new merge request" when you targeted a different target branch when editing an existing merge request.

Value

Being able to support really large projects, with advanced git flows, like the linux kernel git workflow