Refactor topic models (bigun)

See 0117 for a good overview.

ProjectTopicContainers (god what was I thinking) => Topic
Topic => LegacyTopic
TopicNodeAssignment => TopicAssignment

Lots of refactoring in any file that referenced anything about topics.
Merge branch 'master' into change-topic-models

Conflicts:
	editorsnotes/main/views/notes.py
Merge pull request #163 from editorsnotes/change-topic-models

Refactor topic models (bigun)