Dhm/filter special

@cahrens @sarina @cpennington Can one xblock person + cahrens review this for https://edx-wiki.atlassian.net/browse/STUD-683?filter=-1 ? 

btw: it was always computed rather than read; so, it caused no errors before.

btw2: do I need to provide a reviewable mongo query to remove this from all existing entries?