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)
      - DiffusionBrowseDirectoryController (concrete, handles directories)
      - DiffusionBrowseFileController (concrete, handles files)
      - DiffusionBrowseSearchController (concrete, handles search)

Feels a lot cleaner.

Test Plan: Looked at directories, searches, and files.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7045