Will/bok choy test robustness

Fixes for some failures in the bok-choy test suite I've observed locally:

* Add page reload retry for finding course fixture on the "find courses" page.  The logs indicate that the course was successfully created; the screenshots confirm that other courses were available.  This suggests that there is a race condition between course creation (in Studio) and course availability (in the LMS).  My solution here is to reload the page up to 3 times if the course isn't found, retriggering a query to Mongo.

* Protect Acid XBlock tests from `StaleElement` exceptions (by using `map` instead of accessing Selenium elements directly)

* Allow Acid XBlock test to retry if it can't find any component locators, likely because the JavaScript hasn't finished updating the page.

@jzoldak