Handle robotext in the message's title

Summary: Handle robotext in the message's title - we need to pass the user's name and replace 'you'/'your' with it in a robotext.

Test Plan: create sidebars out of robotext messages and check if the titles are correct for everybody

Reviewers: ashoat, palys-swm

Reviewed By: ashoat, palys-swm

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D714