Customize the Session admin

[jrrickerson]

Customize the Session admin to make it easier for the group administrators to use. Consider the following enhancements:

• Configure listing fields to provide relevant information at a glance
• Consider adding a calculated list field to show the number of talks booked for a session (to show a group admin where he may need to add talks)
• Configure filters to be able to filter sessions by the number of approved talks
• Consider adding an Inline to the Session admin to display the list of Talks available, with a link or button to the Talk admin to approve that Talk for the Session.
• Configure the queryset to list Sessions by date.