Login page

[sergeysova]

• Create a new client application in the authmenow
  auth-dev.atomix.team/admin/auth/callback, scope: 'admin:root'
• Create a page with a single button [ Login ]
  it should redirect to authmenow/oauth/authorize
• Create a page /admin/auth/callback that call backend
  to exchange an access token and get info about the current user
• Handle errors:
  • "Registration is not allowed"