Selenium (part 1)

First of several happy-path Selenium tests for [ORA-659](https://openedx.atlassian.net/browse/ORA-659).
This tests the self-only flow -- I plan to add more tests in the next few days, using this as a starting point.

@stephensanchez @jzoldak Please review.
I think we should avoid assertions in page objects as per bok-choy [guidelines](https://github.com/edx/bok-choy/blob/master/docs/guidelines.rst#put-assertions-in-the-test-not-the-page-object)