mentoring: Allow partial answers when dependency isn't enforced

@sarina This is a small update of the mentoring XBlock for the Harvard GSE1.1x course. The change has been requested by the course content team, to be more permissive with content entered the students and always accept answers (green checkmark) on questions for which dependencies aren't enforced.

The diff: https://github.com/gsehub/xblock-mentoring/compare/d4532e4f89aaf36b56715be2abc0f9402912794e...4d1cce78dc232d5da6ffd73817b5c490e87a6eee