Adding topic list on index page.
When a question is marked as topic, it's promoted before the question list
and "no answers" message is hidden.
- Top Questions doesn' show questions marked as topics anymore.
- Fixed bad indentation.
Added set_topic mod in mod bar to allow user set or unset the question as topic.
Removed has_topics variable from index.html template
Merge branch 'topics'

Conflicts:
	knowledge/templates/django_knowledge/index.html
	knowledge/views.py