Rename Event "userPHID" to "hostPHID"

Summary: Ref T10909. Ref T9224. We label this field "Host" in the UI; make the storage format consistent.

Test Plan:

• Viewed month view, day view, detail view of an event.
• Created a new event, saw myself as the host.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9224, T10909

Differential Revision: https://secure.phabricator.com/D16291