Django Admin-integration: Make filter handling more intuitive

[maennel]

Add the possibility to clear the filter on a model's view (currently one can't return to the initial view, listing ALL objects and not only filtered ones by either the as_of or the is_current filter)

[boydjohnson]

I've looked at it and it seems that one can toggle between is_current and all, but once an as_of filter has been applied there isn't a way to clear it and the clear button doesn't seem to clear anything.