Use ApplicationSearch in ReleephBranchView

Summary:
Ref T3721. Releeph currently attempts to implement a flexible, field-driven search for branches, but it's building all of its own infrastructure and it ends up heading down some weird paths. In particular, it loads **every** request and then makes calls into fields to filter them. It also tries to be very very general, which isn't really necessary (for example, I think it's reasonable for us to assume that we won't let you disable the "requestor" field).

ApplicationSearch and CustomField provide more scalable approaches to this problem; move search on top of them. The query still ends up doing some filtering in-process, but it's now far more limited in scope and can be denormalized later.

Test Plan: {F54304}

Reviewers: btrahan

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T3721

Differential Revision: https://secure.phabricator.com/D6758