Resolves #40 "Allow judges to be assigned to multiple areas"

Closes #40 (closed)

Updates models to accept multiple areas using a JSONField in changes-for-judge-multi-assignment. Updates REST endpoints, tests, and frontend accordingly. Adjusted REST endpoint for local authority retrieval to omit the Unknown authority. Updated school for local authority REST to take multiple + separated local authorities. Also updated the frontend handling of retrieving the schools associated with the chosen local authorities to use Unknown if not local authorities are currently selected (upon page load and clear of the Select).

Opted not to extend to multiple schools as it seems unnecessary due to Judge models only having an area attribute.

Demo