Distinguish "formatted" from "unformatted" post revisions, and do
not bother storing HTML for unformatted post revisions.

Used currently for topic title posts.

Change-Id: If45b5a35b49a6d2087e0e2eaf479549418b63c64
Merge "Distinguish "formatted" from "unformatted" post revisions, and do not bother storing HTML for unformatted post revisions."