Allow users to edit a proposed talk

[jrrickerson]

Currently, an end user can only submit a talk.
Once the registration system is in place (see #19) and we can authenticate a user, modify the talk detail view and provide an "edit" link to allow a user to update the talk's description and title. If the talk has not yet been booked for a specific session, allow the user to edit the available sessions as well, but if the talk has already been booked the user cannot edit the available dates.