Change namespace for added admin templates

Having one directory for each model seems like overkill; per-app
namespacing is enough.
Add front-end code for ordering topics in a lesson

It was necessary to change the custom Topic manager into a custom
LessonTopicRelation manager. Because lessons and topics have a
many-to-many relationship, a topic obviously doesn't know which lesson
it's ordered in relation to; a lesson-topic relation, on the other hand,
sees all.