Authentication: Add roles support

[xerbalind]

Zauth has recently implemented roles (see ZeusWPI/zauth#328).
Every board member has bestuur role. So you can check for this role during authentication.

I have also already created the tabdmin_admin role and added ieben to it. So you don't have to hard code your username 😄.

So during authentication, you just need to add the roles scope and the roles will be included when fetching the user info.