Ensure an Approved Talk is only assigned to a single session

[jrrickerson]

While a speaker may choose several sessions he is available to give a talk, once the talks administrator approves a talk, it should only be approved for a single session, and should be considered "booked" unless it is canceled at some future point.

• Build a mechanism to book talks for a single sessions (might need a new linking model between Talk and Session. Maybe Booking?)
• Ensure that either all other sessions linked to the talk are removed, or, if we want to avoid removing data, leave the available sessions and create a new link to specifically indicate the booked session.
• Unauthenticated list views of talks should only displayed booked talks for a session.