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!)
This closes #116

* Add new placeholder entries on TextArea, TextBox and Multi-element
TextBoxes.
* Add placeholder fields in Object_Talk and Object_User templates and
classes.
* Amends Generic_Object class to include placeholder texts
Show the overlap flag on the personal timetables even when not logged in

As per Jack's suggestion.
Merge pull request #170 from JonTheNiceGuy/Issue120

Adds double booking flag to close issue #120.