Wait for element to not be present. (Compatible with minor bok-choy upgrade.)

With an upcoming bok-choy upgrade, 'not-present' and
'invisible' are two distinct concepts. In the upgrade, an
invisible element must be present on the page. This commit
draws that distinction across two helper methods, one for
'invisible' and the other for 'not-present' (or, not on the page at all).

common/test/acceptance/tests/studio/test_studio_course_team.py

@@ -42,7 +42,7 @@ class CourseTeamPageTest(StudioCourseTest):
     def _go_to_course_team_page(self):
         """ Opens Course Team page """
         self.page.visit()
-        self.page.wait_until_ready()
+        self.page.wait_until_no_loading_indicator()
 
     def _refresh_page(self):
         """

common/test/acceptance/tests/studio/test_studio_library.py

@@ -523,7 +523,7 @@ class LibraryUsersPageTest(StudioLibraryTest):
         """
         self.page = LibraryUsersPage(self.browser, self.library_key)
         self.page.visit()
-        self.page.wait_until_ready()
+        self.page.wait_until_no_loading_indicator()
 
     @flaky  # TODO fix this; see TNL-2647
     def test_user_management(self):