Fix API inconsistencies

Update to unique slugs/removed remote_id

Add Document model

Remove fingerprints

Fix deps

Remove features that don't belong here

Fixed API

- Remove unused imports,
- Use HyperlinkedModelSerializer,
- Hide the internal ID field,
- Hide fingerprint,
- Cover more endpoints with tests,
- Remove as much untested code as possible,
- Clarify which URL is for what resource (ie. ProposalDetail didn't list
  votes but ProposalList did, ending up in horrible performance),
- Change tests so that they query indented responses, making diff'ing
  easier

Index Proposal.datetime

It's used to sort Votes, takes votes list of 10 items in the API from 5s
to 300ms.

Add import from FranceData