Added test_clicking_on_content_rating and removed hardcoded test app

This is for issue https://github.com/mozilla/marketplace-tests/issues/525

Also made some changes to the Details page, not to use hardcoded test app
This same code already exists in `test_search.py` [1], so this is duplication. I suggest adding it to `base_test.py` [2] and then using that for both `test_search` and `test_details_page`.

[1] https://github.com/mozilla/marketplace-tests/blob/master/tests/desktop/consumer_pages/test_search.py#L18
[2] https://github.com/mozilla/marketplace-tests/blob/master/tests/desktop/base_test.py