Added initial storage structure for Phrequent.

Summary:
Added the initial storage structure (DB tables and DAO classes)
for Phrequent.

Test Plan:
Apply the patch and run `bin/storage upgrade`.  It should
complete successfully.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2857

Differential Revision: https://secure.phabricator.com/D5476