Fix PositionTitle's get_absolute_url()
Fix PositionTitle's get_absolute_url()

This was broken by the change to the naming of the position
views, and meant, for example, that the autocomplete in the
search view would return 500 errors with the term "memb".

hzj123/56th,geoffkilpin/pombola,hzj123/56th,mysociety/pombola,hzj123/56th,geoffkilpin/pombola,geoffkilpin/pombola,mysociety/pombola,patricmutwiri/pombola,hzj123/56th,patricmutwiri/pombola,patricmutwiri/pombola,mysociety/pombola,geoffkilpin/pombola,hzj123/56th,mysociety/pombola,geoffkilpin/pombola,mysociety/pombola,geoffkilpin/pombola,ken-muturi/pombola,hzj123/56th,ken-muturi/pombola,patricmutwiri/pombola,ken-muturi/pombola,patricmutwiri/pombola,patricmutwiri/pombola,mysociety/pombola,ken-muturi/pombola,ken-muturi/pombola,ken-muturi/pombola