R1:176ee9a889df

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…