Verify required version branches exist

In order to speed up things getting through integration we should add a check to ensure all bugs are backported to current supported branches.

The important goal of this is that the developer has expressed their intention, not that we enforce it with the pre-checker. For example - the developer could explain that a bug doesn't apply to an older branch, or they could justify in some other way why it doesn't apply.

In order to achieve this we'd need some way of the developer acknowledging a missing branch.'

Me & Eric had a quick chat about it: https://moodle.org/local/chatlogs/index.php?conversationid=15958#c557565