in doing so, applies the improved error message to mixins as well (previous commit only applied it to decorators)

changes failure on request.session.get('LTI_LAUNCH', None) returning None from an AttributeError to an ImproperlyConfigured exception

* develop:
  moved the code that stashes the LTI launch params to be after login(); fixes issue with masquerading in Canvas

moved the code that stashes the LTI launch params to be after login(); fixes issue with masquerading in Canvas

Make corresponding mixin for LTI roles being required.  Remove GroupRequ...

Make corresponding mixin for LTI roles being required.  Remove GroupRequiredMixin and LoginRequiredMixin since the former is not used in LTI requests and neither was being used currently anyway.  Require django_braces with this library and use it's LoginRequiredMixin instead.  Update version.

when authenticating the user, put all of the LTI launch params into a dict called LTI_LAUNCH in the session

middleware will process all LTI launch requests, regardless of whether or not the user is already logged in.

when extracting the userid from the LTI launch request, look for lis_person_sourcedid first, then custom_canvas_user_login_id, then user_id

don’t throw PermissionDenied if there is no LTI data in the request; this is to allow view-by-view authentication

Handle case where the SIS user id isn’t available in the LTI request; handle missing email/first_name/last_name