Add label merging

Problem to solve

I have a project with two labels: Label A1 and Label A2. I decide to only have Label A1 but some things are already tagged with Label A2. If I now delete Label A2, all my issues tagged with it will lose the label without gaining a label on Label A1.

User experience goal

The user should click the menu next to the Edit button on the label management page in order to access a new Merge into... sub-menu. Perhaps similar menus should exist group-wide and instance-wide.

Proposal

The shown sub-menu should bring up a small dropdown populated with possible merge targets and perhaps a search. Upon merging, all issues and MRs currently tagged with the old tag should receive the merge target tag after which the now-unused original tag is deleted.