Rename PhabricatorUserStatus to PhabricatorCalendarEvent

Summary: Ref T4375. We never join this table, so this is a pretty straight find/replace.

Test Plan: Browsed around Calendar, verified that nothing seemed broken. Saw my red dot in other apps.

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4375

Differential Revision: https://secure.phabricator.com/D8145