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 to give admin rights.

An additional role haldis_admin can also be created to give specific users admin rights.

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