Change text of registration to enrollment

[ECOM-463](https://openedx.atlassian.net/browse/ECOM-463)

Reviewers: @dianakhuang @srpearce 

@sarina When is the best time to merge this relative to the release process to avoid breaking translations?
@sarina The marketing site "force language" tests require dummy strings for the enrollment button.  Is there a better way to ensure that this string is available?