Conpherence - make adding participants be a little dialogue.

Summary: and now you can add more than one at a time! Also adds the 'add participants' and 'new calendar event' options to mobile view. Fixes T3251. Ref T3253.

Test Plan: loaded up these "adders" on both desktop and device-ish views and it went well!

Reviewers: epriestley, chad

Reviewed By: chad

CC: chad, aran, Korvin

Maniphest Tasks: T3251, T3253

Differential Revision: https://secure.phabricator.com/D6075