Provide generalized custom field storage and custom field indexes in Maniphest
Summary: Ref T418. Depends on D6992. This adds index and value storage for Maniphest custom fields.
Test Plan: Ran storage upgrade.
Reviewers: btrahan
Reviewed By: btrahan
CC: aran
Maniphest Tasks: T418
Differential Revision: https://secure.phabricator.com/D6995