Support for automatic ~workflow::verification labelling of the merged MRs

Problem to solve

At the moment, developers add Closes xxx to their MRs and this automatically closes the corresponding Issue when the MR gets merged. Updated workflow process assumes that now, we do not automatically close the Issues but rather label them with gitlab-ce~11090870.

So, a couple of things should be done:

• we should not close Issue automatically on MR's merge even if it has Closes xxx
• we should either
  • implement a new "keyword" (References XXX maybe?) that would tell process which Issue should be labeled with gitlab-ce~11090870 after the MR got merged, OR
  • re-purpose Closes xxx workflow to not close the referenced issue, but rather label it accordingly