Fix the "transition_to_primary" upgrade.

fixes #3551
Add Selenium and Frontend tests for #3551
Simplify and clarify the Selenium and front end tests.

* To Selenium, simplify the tests.
* To Selenium, add better commenting.
* To Frontend tests, clarify the transition_to_secondary test names to be self explanatory.
Merge pull request #3552 from mozilla/issue_3551-transition_to_secondary

Issue 3551 transition to secondary