Add django checks to detect misconfiguration

[merwok]

This lib is a toolkit with optional features, but when some feature is enabled it requires some settings. We should use the django checks subsystem to issue warnings.

We could do it using AppConfig.ready: https://github.com/fabiocaccamo/django-admin-interface/blob/master/admin_interface/settings.py
or use the django checks subsystem that is used by manage.py check and manage.py runserver: https://docs.djangoproject.com/en/2.1/topics/checks/

• facebook login requires facepy
  

  rest-framework-auth-toolkit/rest_auth_toolkit/views.py

  Lines 183 to 185 in 0bf0fb5

  	if facepy is None:
  	raise TypeError('install rest-framework-auth-toolkit[facebook] '
  	'to enable Facebook logins')

  
  and settings

  rest-framework-auth-toolkit/rest_auth_toolkit/serializers.py

  Lines 76 to 78 in 30aba20

  	req = facepy.SignedRequest(data['signed_request'],
  	settings.FACEBOOK_APP_SECRET_KEY,
  	settings.FACEBOOK_APP_ID)

• email signup with confirmations requires settings

  rest-framework-auth-toolkit/rest_auth_toolkit/views.py

  Lines 73 to 77 in 0bf0fb5

  	else:
  	user = deserializer.save()
  	if self.email_confirmation_class is None:
  	raise MissingSetting('email_confirmation_class')

  
  and templates

  rest-framework-auth-toolkit/rest_auth_toolkit/views.py

  Lines 28 to 32 in 0bf0fb5

  	If the setting email_confirmation_send_email is true (default),
  	the function send_email will be called. That function requires
  	that your project defines defines two email templates:
  	- rest_auth_toolkit/email_confirmation.txt
  	- rest_auth_toolkit/email_confirmation.html