Fixes 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