Clone MR

As a developer working on solving a bug that occurs in multiple (release) branches I want to clone the MR.

This to ensure we have a trial of solving the bug in all relevant branches.

How to use:

1. You have an existing MR (feature1 => master)
2. Press the 'clone MR button'
3. Select the branches you want to clone it to (release1, release2)
4. Two new MR's are created (feature1 => release1 and feature1 => release 2) with a link back to the original MR and original issue
5. Merge the different merge requests (can maybe automerge if it is a clean merge)
6. The original issue now contains linked to all MR's, showing in which releases it was resolved

/cc @JobV @dzaporozhets @markpundsack

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