Move data/* into centralserver/{app}/data/*

Currently, both `ka-lite` and `ka-lite-central` define `settings.DATA_PATH`.  However, they each have their own separate data repository, so overriding one from the other causes problems.

Data are tied to an app, so they should be packaged into the app.  This change moves data inside `{repo}/data/*` into `{repo}/{app}/data/*`.

I also fixed a couple of new bugs (did not open issues) along the way.

**Testing:**

I did basic server loading (both central and distributed) and accessing to check that #62 and https://github.com/learningequality/ka-lite/issues/1732 generally look good, but nothing close to exhaustive.