Use ApplicationSearch to power primary search

Summary:
Ref T4365. Drive primary search through ApplicationSearch instead of through a bunch of custom nonsense. Notably, this allows you to save searches, notably.

The one thing this doesn't do -- which I'd like it to -- is carry your query text across searches. When you search for "quack", I want to overwrite the query in your default filter and give you those results, so you can turn the search into an "Open Tasks" search by default by reordering the queries. I'll probably do that next. It feels a little hacky but I want to try it out.

Test Plan: {F106932}

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran, bigo

Maniphest Tasks: T4365

Differential Revision: https://secure.phabricator.com/D8123