New activity/session being added, activity date restricted by event dates

[brettski]

The api needs to verify that the dates set on a new activity/session fall within the start and stop dates of that event.

• If the activity is not approved or active it may have no dates
• if the activity is active it must have a startTime between the start and end dates of the event
• Not sure on rule for approved as it may not be scheduled yet, which is fine for now.

related to THAT.us issue: ThatConference/that.us#594