R1:9b318e4044c8

Respect username letter case in Remarkup

Test Plan:
Type ##@makinde## to comment, verify that it is converted to ##@Makinde##.
Verify that ##@NonExistent## stays ##@NonExistent##.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1725