Organisation members view improvements 

This commit contains some of the changes discussed in #1255. Specifically:
* A house page now links to current members instead of past members.
* A former office bearer no longer appears on the former members page. This change has involved handling all position related code only in SAOrganisationDetailSub instead of in both SAOrganisationDetailSub and OrganisationDetailSub - which should also make things slightly easier to follow. As noted in the comments there is still room for improvement.
* Handling of the NCOP page has been improved - allowing for delegates to be treated in the same way as members and through the adding of the office bearers filter to the NCOP view.