Prevent a crash when a group has no forum posts in it. Fix up some language strings on the group info page too.

The name is largely irrelevant as it's only the creator of the file.

Ran the copyright notice corrector script so all Mahara related files are correctly licensed under the GPL3.

Changed the add/edit/delete pages to redirect there afterwards.

- base class now has more functions and a few abstract ones
- edit and delete are provided by interaction/, not the plugin (with class functions to return pieforms)
- added deleted flag and creator (fk to usr) on the interaction_instance table
- added a README about how to write a new interaction plugin