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".
Allow changes to position order on the organisation' people page

This also changes the default ordering to be by the legal_name
of each person, which makes more sense as a default, I think.

This is part of the fix for #659.