add an API for Pombola data

[mhl]

The API help page on Pombola sites that have POPIT_API_URL suggests using that PopIt as the API for Pombola data; however, we're retiring PopIt now so need an alternative API to let people programmatically use the useful data in Pombola instances. This is also something that @idesouza would like for Odekro.

Using django-rest-framework seems like a good basis for this - starting with the models in core, and then hansard next since providing structured data of debates is one of the key use cases that @idesouza mentioned. (Ideally we would migrate hansard to SayIt, but that's not a short term plan because of the time it's taking to do #1594, so it makes sense to do this anyway, I think.)

Another thing to consider is whether we want to require registration for use of the API key (or at least make that an option). It was certainly awkward for PopIt that we didn't have a way of contacting all the people using an instance.

[mhl]

Ian indicated the first priority is to have hansard sittings and proceedings available over the API.

[mhl]

Note from the sprint meeting: timebox to 1 day initially, and get feedback