Extract out bedevere/util.py as shared lib

[Mariatta]

For miss-islington, I'm finding myself needing to write the same code that's already in bedevere/util.py, specifically normalize_title(), is_core_dev(), issue_for_PR() methods.

Wondering if we want to extract those as a shared library for the bots.
Wanted to get your thoughts about this @brettcannon.

[brettcannon]

Fine by me!