Cleanup merged/closed branches via `@rustbot glacier`

[JohnTitor]

Noticed in #610 (comment).
The glacier command is really nice but the merged/closed branches won't be deleted. This is a very minor issue but ideally, it'd be great if we could cleanup somehow.

[camelid]

Note that the branches are in triagebot's fork, not rust-lang/glacier.

[camelid]

cc rust-lang/triagebot#1041

[JohnTitor]

Note that the branches are in triagebot's fork, not rust-lang/glacier.

Yeah, therefore we cannot do it manually after merging.

[camelid]

Maybe we can get triagebot to "check" the "Allow maintainers to edit branch" option?

[JohnTitor]

IIRC it won't work after merging.