Add new central authorization/authentication

[leepeuker]

We currently have only a pretty bare-bone authorization and authentication setup, which shows its issues adding the new REST Api authentication.

Proposed solution is to implement OAuth2.

I suggest the php lib from thephpleague (server and client). Documentation can be found here.

Todos:

• [ ] add OAuth2 authorization to REST api
• [ ] remove manual generated api token from user settings again
• [ ] adjust documentation
• [ ] use OAuth2 in web frontend and replace the current token system

[slimshizn]

Would love to see this implemented.