Block logging for users without any group assigned - frontend

[monikakrawczak]

We have few admin users who shouldn't be able to log in as a driver or passenger to the regular frontend as this doesn't make sense. Let's block them from logging in by checking if said user has some is in Passengers or Drivers group.

Backend is already merged - #413

What we need to do now is:

1. When a user is not assigned to any group:

• block it and display the error message:
  "Logging in is not possible.
  If you want to log in, ask the admin to add you to the group of Passengers or Drivers"
• add button "OK" -> which closes the error message

[arturtamborski]

still relevant, backend should not issue tokens for users who do not have groups assigned to them.