R1:0139fb9178a7

Improve organization of Diffusion browse controllers

Summary:
Currently we have this:

- DiffusionController (abstract, has some random shared browse code)
- DiffusionBrowseController (concrete, Handles routing, directories, and search)
- DiffusionBrowseFileController (concrete, handles files)

Instead, do this:

- DiffusionController (no browse-related code)
- DiffusionBrowseController (abstract, shared browse code)
- DiffusionBrowseMainController (concrete, handles routing)…