Merge and modernize Browse controllers in Diffusion

Summary:
Ref T4245. Browsing is huge and currently split across 5 files using controller delegation.

Although having a huge file isn't great, I think the way it is split up is currently worse, and it gets weird with more flexible repository identifiers.

So this is mostly merging five controllers into one, then a bit of modernization.

I think this can probably be split up better by pulling some of it out into views, instead of using delegation.

Test Plan: Browsed files, directories, and search results.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T4245

Differential Revision: https://secure.phabricator.com/D14942