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?
Is there a way to make "Pending Instructor Tasks" bold? If not, I'd go with double quotes instead of single. Also, I'd change "section" to "section below" to orient the reader.