Consolidate the methods in the mixin, and removed repeated get for DRY.

This commit consolidate as many as possible method in the Category Mixin and in the Participant Mixin.
Swaps also the order for the class used to construct the View, using (Mixin, View) to make sure it's inherit the right way.
Merge branch 'master' into mixing_consolidation
Fixed the name of the class in the super call in the queryset
Merge pull request #89 from e-Luminate/mixing_consolidation

Mixing consolidation.