Add committee filters to PositionQuerySet
Add committee filters to PositionQuerySet

This decides whether a position is on a committee by looking for
the word 'committee' in the organisation's slug, which may prove
fragile.

patricmutwiri/pombola,geoffkilpin/pombola,patricmutwiri/pombola,patricmutwiri/pombola,patricmutwiri/pombola,ken-muturi/pombola,patricmutwiri/pombola,ken-muturi/pombola,mysociety/pombola,geoffkilpin/pombola,ken-muturi/pombola,mysociety/pombola,patricmutwiri/pombola,mysociety/pombola,geoffkilpin/pombola,mysociety/pombola,mysociety/pombola,ken-muturi/pombola,mysociety/pombola,geoffkilpin/pombola,geoffkilpin/pombola,geoffkilpin/pombola,ken-muturi/pombola,ken-muturi/pombola