Reconfigure stalebot

[rowanmanning]

Stalebot helpfully closed all of the issue while we were away over Christmas. We've decided on some new rules that we should implement to stop this from happening to issues we care about:

Stalebot should:

• use a label other than wontfix, it's confusing
• not mark issues as stale if they have the help wanted or good first issue labels
• not mark issues as stale if they have been assigned to somebody

(You should start by reverting commit 06ae99f78904e69d7f68a18a079e80cd9d79f589, which removes Stalebot entirely)

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[rowanmanning]

Reopening, we probably should have disabled stale bot while we were on hiatus.