Import refactoring

Fixes #1767 and #1768.

There are no functional changes here (hopefully!); this is a bunch of import tweaks:
* Make imports within an app relative (so that moving an app name / location would be easier)
* Make imports across apps use the `kalite` prefix (so that name collision within centralserver can be avoided, and because it "just makes sense")
* Reorder imports, based on the above changes, to follow our import conventions: https://github.com/learningequality/ka-lite/wiki/Coding-guidelines-and-conventions#imports
* Remove paths (from `manage.py` and `ka-lite.wsgi`) that allow kalite imports without the `kalite` prefix.

Testing:
* After each commit, I tested some relevant URL endpoints to make sure no gross errors were committed.
* I tested that a new install works fine
* I tested that new migration files continue to use the existing naming conventions.
* I did not test tests, as I don't believe they're working well anyway.  Note that much of the import tweaks were in tests.

@aronasorman, got 5 mins to review the basic idea of what I'm doing here?