Switch from hand-coded CGI to `proper REST framework'

[MichaelClerx]

Eventually we should have it auto-generated with swagger - pending improvements to fc-runner to use a proper REST framework rather than hand-coded old-fashioned CGI.

• Investigate best REST framework to use. django-rest-framework is an option, but may be overkill! Unless we treat it as a learning exercise in preparation for doing the same with the front-end, anyway. Ideally something that can serve via wsgi so is consistent with the front-end.

[jonc125]

Possibly https://falcon.readthedocs.io/en/stable/, which has community add-ons for api doc generation, and an example of use with Celery.

[MichaelClerx]

Certainly looks stable!