Create merge request via email with branch name

We already have a 'create patch' function in the UI.

It would be convenient if you can also upload a patch file and create an MR from that.

Reports:

• https://forum.gitlab.com/t/apply-patch-using-web-ui/5452
• https://lists.archlinux.org/pipermail/arch-dev-public/2017-May/028860.html

/cc @victorwu

Flow

• Similar to a flow where pull / merge requests are mediated through email: https://www.kernel.org/doc/html/latest/process/submitting-patches.html.
• Create a remote branch from the project's repo (or a fork of the project) on your local machine.
• Push changes to the remote branch.
• Send an email to a special email address. Indicate the branch in the email.
• GitLab automatically detects the branch in the email and creates a merge request.
• GitLab responds back to the author (via email), with a link to the merge request.