Make event hosts editable

Summary:
Fixes T10909. I think this is a generally reasonable sort of capability to expose, although I've made it edit-only for now (when creating an event, you're always the host).

Also clean up some minor leftovers in the code, and a couple of little bugs with recurrence frequencies.

Test Plan: Created an event, edited the host of an event.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10909

Differential Revision: https://secure.phabricator.com/D16292