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?
Not sure what "define automated student assignment to groups" means - could this be "define the ways students are automatically assigned to groups"?