refactor default proposal sort

I just noticed that I missed some things while doing #860: There are actually quite some factors that influence the default proposal pager sorting. Users con configure an individual default and is behavior. So I refactored the code to not ignore those factors.

I also changed the code so that `adhocracy.listings.instance_proposal.sorting` does not only influence the proposal index page but all proposal pagers (e.g. the one from #865).

Finally I changed the user profile settings so that one deselect a default proposal sorting. `user.proposal_sort_order` is actually `None` by default. But once set, it could not be unset from the UI.

We should remember when testing this feature that `adhocracy.listings.instance_proposal.sorting` can be overwritten by user specific settings.