Minor enhancements: Allow admin's docs, use with_timestamp and with_author decorators
Enable comments app
Improve ProblemAdmin
Add Meta information to models
Remove Gallery model
Add capability to determine last event where the problem was used
Improve last event summary for problems
Add capability to upload Leaflets
Adds a comment section to the change_view of the ModelAdmin

This can be triggered using the admin_commentable decorator
created in the base/util.py

Also, to allow this functionality, it was necessary to override
base template of the admin site, taken from grapelli.
Move ModelAdmins to admin.py, improve ProblemSetAdmin