Add API authentication helper.

This is a small helper service that takes Basic Auth credentials and turns them into custom HTTP headers that an API can look at.

Eventually this will grow to support OAuth credentials instead (but probably not in this PR).
The difficultly of using async stuff with oauthlib is the reason I stopped
trying to do this auth store in redis. We're going to have to likely
re-write ``verify_request`` when we switch to async so I'd rather this code
be clean and correct now rather than future proof it against some unknown
code changes.