Prevent MRs from being merged if they are not for the current milestone

Right now, you can prevent a merge request from being merged by starting the title with WIP, which is very useful.

It would be useful to add support for preventing MRs from being merged based on milestones too. Milestones can represent releases and developers may work on issues on future milestones for various reasons. It's nice to have a systematic way to ensure nothing from the future milestones (version) is not merged before the current milestone is finished (next version is released).