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