Add basic edit history to herald rules

Summary:
Add a very basic edit history table to herald rules. This table is updated
whenever saving a herald rule. The contents of the save are not examined, and
the edit history contains no information about the rule itself *yet*. Edit
history can be viewed by anyone through /herald/history/<rule id>/.

Task ID: #

Blame Rev:

Test Plan:
Made a test rule, saved some stuff.

Revert Plan:

Tags:

Reviewers: epriestley, jungejason

Reviewed By: epriestley

CC: zizzy, aran, xela, epriestley

Differential Revision: https://secure.phabricator.com/D1387