Refs #331: Reduce use of LoggerMixin

It turns out most of our remaining uses of LoggerMixin are in
base classes that apps are expected to extend, so we probably
shouldn't take that out until the deprecation period is over.

I did add a bit to the release notes about this, to point out
that code that extends those classes might be using LoggerMixin
indirectly.
Merge pull request #331 from rapidsms/331-dont-use-loggermixin

Don't use deprecated LoggerMixin