Control panel tests

Summary of changes:
* Tweaks to Readme
* Imports of KA-Lite distributed app
* Require zone for Facility, FacilityUser, and FacilityGroup
* Delete add_facility view and change all references to facility_edit
* Move general JS functions from distributed to main on ka-lite subtree
* Import purl.js into base template
* Overwrite khan-lite.js csrf variable
* Automated browser tests (first pass)

Testing:
* Manually tested - all seems to work except group changes on central and distributed not syncing, as detailed in #92.
* Automated tests - all locally defined browser tests pass, except the one designed to catch https://github.com/learningequality/ka-lite/issues/1837 , and the final test that is still not handling the appearance of the modal box properly.
* Sync testing - see manual testing.