title: List filter and search results in the admin
body: <p>After I override the <code>get_search_results</code> method, <code>list_filter</code> is not working but the search field works as I expect.</p>
  
  <pre><code>class NaruceniProduktiAdmin(admin.ModelAdmin):
      search_fields = ('broj_narudbe', )
      list_filter = ('date', 'status', )
      list_display = (
          'naziv_produkta', 'datum', 'narudba_broj', 'boja', 'velicina', 'cijena', 'kolicina',
          'ukupna_cijena','korisnik_link','status','source_link',
      )
      actions = [dostupan, nedostupan, email_dostupan, placen, posalji_racun, poslan, isporucen, storniran, posalji_storno, ]
  
      def get_search_results(self, request, queryset, search_term):
          queryset, use_distinct = super(NaruceniProduktiAdmin, self).get_search_results(request, queryset, search_term)
  
          try:
              search_term_as_int = int(search_term)
          except ValueError:
              search_term_as_int=search_term.strip()
              queryset |= self.model.objects.filter(korisnik__ime__icontains=search_term_as_int)
  
              if not queryset:
                  queryset |= self.model.objects.filter(korisnik__prezime__icontains=search_term_as_int)
          else:
              queryset = self.model.objects.filter(broj_narudbe=search_term_as_int)
  
          return queryset, use_distinct
  </code></pre>
  
  <p>If I remove <code>get_search_results</code>, then <code>list_filter</code> works as expected. I want to keep it because I want the search fields to work how I expect, but I also want the list filtering to work as it normally does.</p>
  
  <p>Is there a way that I can use both of these together? And is there a reason why one influences the other?</p>
  
  <p><img src="https://i.stack.imgur.com/E2Kzw.png" alt="enter image description here"></p>