Adds double booking flag to close issue #120.

* Added new "hasOverlap" boolean to Talk objects
* Adds new grey triangle flag to MainScreen, and a more complete flag on
the Object_Talk and Timetable screens (visible only to workers and
admins on the last two screens). Object_Talk includes a hyperlink to get
you to at least one of the conflicted talks (aside from this one,
assuming multiple conflicts!)