R1:87fa97e49aaf

Require double brackets for Phriction links

Summary:
Single brackets are getting some troublesome false positives in Facebook's
install. Particularly, there's a weird convention at Facebook of tagging diffs
by putting stuff like "[perf]" or "[chat]" in the title, although this isn't
turned into structured data at any stage. When commits appear in Diffusion, we
currently link such ad-hoc tags to Phriction.

Wikipedia uses double-bracket sytnax, as do many other wikis, so this seems like
a reasonable burden…