Allow pausing the merge train

[DanielNoord]

Although we never write buggy code sometimes we need to deploy a hotfix or revert a PR. With the merge train several tags can be build before your hotfix passes CI and joins the queue. Today we hit a record of 7 in between tags, which made it quite hard to see what was up with the queue, especially if the in between tags include manual steps such as database migrations.

It would be good to be able to pause all merges for a project so that only the hotfix can join the queue and be deployed first.