Provide default view and edit policies in Calendar, plus "Event Host" and "Event Invitees"

Summary:
Fixes T9224. This adds:

• A "Default Edit Policy" and "Default View Policy" to Calendar, similar to other applications.
• "Event Host" and "Event Invitees" objects policies.

These policies often end up being redundant (the host can always view/edit, the invitees can always view), but they can be more clear than setting "No One", and "Editable By: Event Invitees" is a legitimately useful policy.

Test Plan:

• Created and edited events.
• Fiddled with defaults.
• Tried to remove myself as the event host for an "Editable By: Host" event, got an error ("you wouldn't be able to edit").
• Tried to remove myself as host/invitee for an "Editable By: Invitees" event, got an error.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9224

Differential Revision: https://secure.phabricator.com/D16294