Added AbsoluteUrlMixin and used it in any model.

cleaned up get_absolute_url() in some models
Merge pull request #1172 from ostcar/get_absolut_url_mixin

Added AbsoluteUrlMixin and used it in any model.
Added a poll description field for each assignment poll

fixed signals in MainMenuTests
Merge pull request #1170 from ostcar/poll_description

Added a poll description field for each assignment poll