R1:1855ed4bee40

Support symbol linking in Remarkup code blocks

Summary:
Trigger the crossreference behavior on code blocks. Limited to
Differential, where we know what the project is, but includes regular
comments, inline comments, and previews of both.

(Hopefully event handlers on deleted elements also get deleted, so we
don't leak memory? Also, caching is a problem, and I didn't find a way
to mark existing cache entries as stale, like
`DifferentialChangesetParser::CACHE_VERSION`...)

Test Plan:
Load Differential…