Give Phriction documents a normal timeline

Summary:
Ref T13077. See PHI840. Ref T1894. I'm planning to just let you comment on Phriction documents. I think this will create a few problems (e.g., around popular documents which collect long comment threads that are eventually obsolete) but nothing should be too terribly critical (e.g., we handle it gracefully when objects have very large number of comments/transactions) and for most documents this is likely just a net improvement.

"Just enable comments" is probably not the final iteration on this, but I think it's probably a step forward on the balance, not a step sideways or a slippery slope down into a dark hole or anything.

Test Plan: {F5877316}

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13077, T1894

Differential Revision: https://secure.phabricator.com/D19659