CR: delete merged branches

[georgiaewhitney]

It's a good habit to keep your branches tidy. If they've been merged, I think it's best to delete and start afresh.

If the feature is very similar/is a refactor of that particular feature, it might be worth re-using, but then you'd have to take extra care to keep your merge up-to-date to avoid conflicts. So, I'd almost always start a new branch.

it's hard to let go sometimes, i know...