Add wait for changing display name, change edit page inheritance

This is to attempt to solve the intermittent failures on saucelabs jobs by adding a wait. It also fixes inconsistencies with which page model is used in checks as well as changes inheritance of edit page to inherit from base. @bobsilverberg r?
I discussed this with @dchunwong and suggested this because this wait is essentially an assert. It's replacing the current assert because that assert was happening too quickly.

I'm not sure how to move it into a page object. The logical place would seem to be to move it into the `click_save_my_changes()` method, so that it will wait until the change occurs, but the problem is that that method is in `edit_page_modal`, which is not the same as `edit_page`.