Over time, djblets.webapi.core has grown, and contained classes better

suited for other modules. We even had a circular dependency due to the

way that things were split up.
This splits out all the encoder-related classes and registration logic

into the existing encoders.py, and all the response classes into

responses.py.
The existing djblets.webapi.core will emit a DeprecationWarning, but

will continue to work as before.
No logic has changed, with the exception of changing old-style

MyParent.function(self, ...) calls to use super() instead.
The only move I wasn't sure of was SPECIAL_PARAMS. I put it in

decorators.py, since the decorators depend on it and are the main place

where API parameters are processed. However, one part of resources.py

also uses it, just to help build a string. I think this move is okay,

but I'm open to ideas.

Djblets and RB unit tests pass. Note that I didn't update the imports in

RB, so this was going through the legacy imports in core.py.