Add deprecated attribute to Field

I would like to be able to mark certain course fields as "deprecated" so that they are hidden by default in Advanced Settings (user will be able to toggle on deprecated settings).

This is useful for fields that were introduced for just one particular course, but now cannot be removed due to reruns of the course.

@cpennington Please review
Already mentioned this in person, but just so the comment doesn't get lost: can you store this in a more semantically named field?