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