Add a new base mixin for LTI views that has two convenience methods for pulling params from session.  Add full_name as a stored session variable.

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

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

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