Stalled branches should be cleaned up

[metacosm]

We have lots of old branches that are not needed anymore. They should be deleted to clean things up. More generally speaking, whenever a PR is closed without a merge, the associated branch should be deleted unless there's a good reason to keep the branch around, in which case, this should be indicated in the closing comment of the PR.

[csviri]

Yep, also we should do PRs from forks to even avoid adding new branches.