Show users how fulltext search queries are parsed and executed; don't query stopwords or short tokens

Summary:
Depends on D17670. Fixes T12137. Fixes T12003. Ref T2632.

This shows users a readout of which terms were actually searched for.

This also drops those terms from the query we submit to the backend, dodging the weird behaviors / search engine bugs in T12137.

This might need some design tweaking.

Test Plan:

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12137, T12003, T2632

Differential Revision: https://secure.phabricator.com/D17672