Adding Canvas.user.loginId to the default list of custom variables

While we only ever needed this for the VitalSource integration,
as a the field to fetch the user reference, it's less error prone
to always ask for it in every new install.

lms/views/canvas/config.py

@@ -11,6 +11,7 @@
     ("custom_canvas_course_id", "$Canvas.course.id"),
     ("custom_canvas_api_domain", "$Canvas.api.domain"),
     ("custom_canvas_user_id", "$Canvas.user.id"),
+    ("custom_canvas_user_login_id", "$Canvas.user.loginId"),
     ("custom_display_name", "$Person.name.display"),
     ("custom_context_id_history", "$Context.id.history"),
     ("custom_course_starts", "$Canvas.course.startAt"),

tests/unit/lms/views/canvas/config_test.py

@@ -49,6 +49,7 @@ def test_it(self, pyramid_request):
                 "custom_canvas_course_id": "$Canvas.course.id",
                 "custom_canvas_api_domain": "$Canvas.api.domain",
                 "custom_canvas_user_id": "$Canvas.user.id",
+                "custom_canvas_user_login_id": "$Canvas.user.loginId",
                 "custom_display_name": "$Person.name.display",
                 "custom_context_id_history": "$Context.id.history",
                 "custom_course_starts": "$Canvas.course.startAt",