Central refactor

This codedepeds on my `securesync-refactor` branch and goes hand-in-hand with the new project: https://github.com/bcipolli/ka-lite-central

KA Lite is an application that is a stand-alone project.  The central server and website aspects of KA Lite are separate components that are value-add / optional features within KA Lite, requiring internet access.

In order to simplify the KA Lite project structure and enable a more flexible relationship between the two projects, I've:
* Removed central-only apps from KA Lite
* Created a new project, `ka-lite-central`, which includes the `ka-lite` project as a subtree, and selectively imports modules needed for the central server.

What remains (as defined in https://docs.google.com/a/learningequality.org/document/d/1K7elIr9eSbzBJrkloMG1LRta-VgDQE6Y98sJ-zRO9l8/edit#):
* Separate not just the code location, but code (i.e. remove the settings.CENTRAL_SERVER flag via further refactoring)
* Separate central server khanload parts out of ka-lite and into khanload_central
* Separate central server i18n code out of ka-lite and into i18n_central
* Migrate to explicit imports of `kalite.[kalite-module]` for central server imports
* Delete central-only static files from `static`

Note: I am submitting this PR now, rather than after doing the above refactorings, to avoid super-enormous merge conflicts and to allow this to happen iteratively.  Doing this all at once is not a feasible thing, but moving the files resolve the most major possible merge conflicts.

Note also: once we discuss and if this gets merged, I will change ownership of the `ka-lite-central` project to `fle-internal` and update the subtree here.