No central content root

This resolves #1192 by scrubbing the code with the following principle:

>All apps should reference shared.topic_tools rather than main.topicdata, for fewer cross-app dependencies (and to avoid the main-specific, big computations in main.topicdata)

`CONTENT_ROOT` and `CONTENT_URL` are distributed server concepts; as the goal is to separate central and distributed servers, and to define/maintain boundaries between Django apps, this cleanup defines lines and makes the code follow them.

Note new issue #1231, which will allow `CONTENT_ROOT` AND `CONTENT_URL` to avoid being even defined for the central server.