Invitee list on Calendar event detail view should have pretty icons

Summary: Closes T8020, Invitee list on Calendar event detail view should have pretty icons.

Test Plan: Open event, invite users, event detail should show invitee names with blank circles, after users accept/decline, icons should update to green check/red times icons, respectively.

Reviewers: #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T8020

Differential Revision: https://secure.phabricator.com/D12720