Use delegation to generalize application search controllers

Summary:
Ref T2625. Lifts almost all of the search logic out of Paste controllers and into Search.

This uses controller delegation for generalization. We use this in a few places, but don't use it very much yet. I think it's pretty reasonable as-is, but I might be able to make even more stuff free.

There are some slightly rough edges around routes, still, but I want to hit Phame and Differential (which both have multiple application search engines) before trying to generalize that.

Test Plan: Executed, browsed and managed Paste searches.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2625

Differential Revision: https://secure.phabricator.com/D6073