Consumer pages sanity tests run

This PR is for issue #524

Tests are strengthened/modified, new tests added to address https://moztrap.mozilla.org/runtests/run/57/env/112/ list with sanity tests


I updated this according reviews from PR https://github.com/mozilla/marketplace-tests/pull/527
I wonder if we should consider making these all into a list of locators, much like `footer_links_list` above. Then we could save creating all of these properties, and also the test wouldn't lose coverage on a failure. Currently, if `is_develop_apps_button_visible` is false then the test will just fail and none of the other items will be checked. Perhaps it would be better to loop through all of the elements we expect to be visible and add a failure message for each one if they are not visible, just like we're doing for bad links.

What do you think, @AndreiH?