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?
@srpearce can I please request that you don't make comments on the changes in these .po files? This string wasn't actually changed in this PR and I'd like to avoid needlessly breaking out-of-scope translations. Please only reveiw the files that are NOT translation files.