[web] Avoid Chrome+Mac sans-serif emoji bug

Summary: For some reason, Chrome can't render ☺️ properly when sans-serif is specified anywhere in the font-family. We can avoid this issue with ui-sans-serif. I also removed some custom font-family that should fall back to the same font-familys specified in other selectors.

Test Plan: tested MessagePreview, ChatInputBar and Entry textareas, emojiOnly messages, and normal messages on Chrome, Firefox, and Safari

Reviewers: atul, palys-swm

Reviewed By: atul

Subscribers: KatPo, zrebcu411, Adrian, subnub

Differential Revision: https://phabricator.ashoat.com/D957