"Draft" or "Proposed" events

[aaronpk]

Create a setting that makes events created by non-admins "draft" or "proposed", and need to be reviewed before they are publicly visible. This is to allow a relatively broad audience to add events where every event needs to be reviewed by a moderator before it's posted to reduce spam and avoid other unwanted events.

Some implications of this:

• A user needs a view that shows a list of their proposed events awaiting moderation
• Admins need a view of all events awaiting moderation
• The "add an event" page needs some text explaining that events are moderated before being publicly visible

This implies some changes to how editing works as well:

• Users can only edit their own events
• Edits need to be approved too, so the edits should go into the moderation queue as well. Admins reviewing the queue should be able to see whether an event is new or being edited.