Demo Mode updates / JS library version updates:
- Use unittest asserts in some places to avoid extra method calls
- Use the correct Demo Mode message for assert_exact_text()
- When checking for JS errors, give enough time for them to appear
- Update Backbone JavaScript library version
- Update Tether JavaScript library version
- Add Demo Mode actions for assert_partial_link_text()
- Add Demo Mode notifications for self.assert_no_404_errors()
- Add Demo Mode notifications for basic assert statements