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