Github: assign a milestone to a pull-request when it is merged
Based on the civicrm-version.txt (?) of a given branch, when the pull-request is merged, something should automatically assign the PR to the given milestone (and possibly create the milestone if it does not exist).
Example: the version on master is 5.4.0, when a pull-request on master is merged, then assign the 5.4.0 milestone to that pull-request.
If the pull-request was done on a branch of an RC, then it needs to assign the milestone for that branch.
Can this be done with botdylan?
Required for #817 (moved)