Fix {Fnnn} rule in Remarkup

Summary:
Remarkup rule callbacks now get SafeHTML matches instead of string matches. If they call:

  $some_lisk_dao->load($matches[1]);

..as is the case with the `{F123}` rule, we reject the SafeHTML as an invalid ID and return null.

Allow load() to string convert any object (which will either succeed or fatal in an obviously-broken way).

(Long ago we threw instead of returning null here, but it meant we had to do a lot of redundant checks.)

Test Plan: `{F123}` shows an image again. `{C1}` embeds a countdown.

Reviewers: vrana, chad

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D4961