Add Ferret support to Paste

Summary:
Ref PHI1292. Enable fulltext searchs in paste. Maybe this should only index a snippet instead of the entire content?

Also updates table names in PhabricatorPasteQuery.

Test Plan: Created some pastes, indexed them, searched for them.

Reviewers: amckinley

Subscribers: codeblock, Korvin, PHID-OPKG-gm6ozazyms6q6i22gyam

Differential Revision: https://secure.phabricator.com/D20650