Convert functions in utils into decorators

[janine9vn]

Context

Decorators are cool, let's use more of them. The _has_role_check() function in the utils folder should be turned into a decorator and then properly applied to the functions it's currently in.

[twavv]

I think it'd be nice to have decorators that handle the membership check as well as to auto-delete messages in public channels, since that seems to be something we do in at least two places.