Branches build up in student directories about abc-classroom

[betatim]

I think we could delete old branches when we make a new PR. Important to check if a branch has been merged (or the corresponding PR closed) before deleting them.

My feeling was that using the template repository as the source of the PR would be trickier. The thinking was:

• we still need a way to clean up
• we need to generate a diff/PR that brings that student's repo up to date. It wouldn't be the same for all students (for example if you didn't merge last weeks PR you need a different one this week compared to someone who did merge it)
• we need to add lots of remotes to the template repo (one for each student)

So I'd investigate automatically cleaning up old/stale/unused branches when we create a new one.