Provide more context to calls to Segment.

Implementation for DE-1089.

Centralize the definition of context into a single method.  This is in
common/djangoapps/track because the context is originally set there by
middleware.

common/djangoapps/student/tests/test_activate_account.py

@@ -72,7 +72,7 @@ class TestActivateAccount(TestCase):
         LMS_SEGMENT_KEY="testkey",
         MAILCHIMP_NEW_USER_LIST_ID="listid"
     )
-    @patch('student.models.analytics.identify')
+    @patch('student.models.segment.identify')
     def test_activation_with_keys(self, mock_segment_identify):
         expected_segment_payload = {
             'email': self.email,
@@ -98,16 +98,16 @@ class TestActivateAccount(TestCase):
         )
 
     @override_settings(LMS_SEGMENT_KEY="testkey")
-    @patch('student.models.analytics.identify')
+    @patch('student.models.segment.identify')
     def test_activation_without_mailchimp_key(self, mock_segment_identify):
         self.assert_no_tracking(mock_segment_identify)
 
     @override_settings(MAILCHIMP_NEW_USER_LIST_ID="listid")
-    @patch('student.models.analytics.identify')
+    @patch('student.models.segment.identify')
     def test_activation_without_segment_key(self, mock_segment_identify):
         self.assert_no_tracking(mock_segment_identify)
 
-    @patch('student.models.analytics.identify')
+    @patch('student.models.segment.identify')
     def test_activation_without_keys(self, mock_segment_identify):
         self.assert_no_tracking(mock_segment_identify)