Calendar upgrades

Summary:
Does a handful of things to make Calendar significantly more useful

- Enabled overlapping events
- Profile has a 'week view' of the user
- Profile has a 'month view' of the users
- Multiple users on a calendar are color coded
- Browse view slightly more useful

This stops short of implementing the new 'home' view on Calendar, mostly this is a big step though to make that happen next.

Test Plan: Make lots of events on diffent users.

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T2897, T4375

Differential Revision: https://secure.phabricator.com/D8317