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