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
Merge pull request #174 from leplatrem/get_rid_useless_tests

Get rid of useless tests
Remove persona dependencies.
Good bye users, welcome tokens.
Add the POST /tokens endpoint to add a new token.
Python 3.4 bytes versus string fixes.
Return 201 Created status.
Update Dockerfile.
@mle review.
@ametaireau review.
Rename hkdf module in tokens.
Merge pull request #178 from spiral-project/user_management

Add POST /tokens — Token creation
Add a Redis backend.