Customize the Talk admin

[jrrickerson]

Build a custom admin class for the Talk model to make it easier for group administrators to use. Consider the following:

• Configure searchable fields so users can search for specific talks by keyword
• Configure filters so talks can be filtered by status or speaker
• Configure queryset to sort most relevant talks to the top of the list (most recently submitted perhaps?)
• Configure listing fields to display the most relevant information at a glance on the list view