The resource definitions for RBTools are quite lengthy. While the bulk of the current implementation all kind of needs to stick together, we can split out individual resource-specific subclasses. The bulk of this is just splitting up code without making any other cleanup or functional changes. The only exception is DraftDiffResource, which was created for a mimetype that doesn't exist in Review Board (the server-side DraftDiffResource reports its mimetype as `diff` instead of `draft-diff`. This resource subclass has been removed and the old name is aliased to DiffResource. Testing Done: Ran unit tests.