Add support for role assignment through REST API

Add an endpoint that allows administrators to add roles to other users.
These roles should be stored in keycloak. So in short, the endpoint:

- verifies that the user has the permissions to change roles of users (is admin)
- verifies the role exists in keycloak
- assigns the role to the user

As long as this feature isn't added, roles have to be assigned through the keycloak interface.