Update Participants UI in Conpherence Full

Summary: This shrinks the UI to fit more people in the participants list, useful for rooms. Also update the remove icon.

Test Plan:
Review a lot of people in a room, so so many.

Reviewers: epriestley, btrahan

Reviewed By: btrahan

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12213