Handle markdown for message title

Summary: This applies the feature of stripping the markdown syntax in messages' previews requested here: https://phabricator.ashoat.com/D666?id=1887#inline-2787

Test Plan: create messages/sidebars containing markdown syntax and see if it's not visible in the main screen of the chat

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D693