Preserve original case in @mentions which whiff

Summary: See T632. When we miss a @mention, preserve the original case. This
approach is slightly unwieldy, but preserves backward compatibility (remarkup is
cached in Differential and Maniphest).

Test Plan:
https://secure.phabricator.com/file/view/PHID-FILE-u7z5j73dxrr4vuwkdcy3/

Reviewers: aran, btrahan

Reviewed By: aran

CC: aran, epriestley

Differential Revision: 1141