Update the server to reflect latest changes.

- Remove the split between database and backends;
- Remove the concepts of users, policies and groups for the ACLs.
Add ACLs management for models.
Fix GeoJSON renderer.
Fix flake8 errors.
@ametaireau review.
Merge pull request #176 from spiral-project/refactor-acls

Refactor ACLS
Get rid of redundant functional tests
No need to vary on views main logics