Add authentication to git server

[EtienneDx]

Current git server is unfinished. We need to connect it to the database to connect user and manage repositories.

Note:

• Public users have read permission for all public repos, but no write permission ever.
• Connected user have the same as public, with an extra read write permission to their repos. At first, let's forget about repo sharing.

[EtienneDx]

Would require coming after #36, as we don't have public keys in users yet.