Closes #12622: Fix assigning VLAN without site to Prefix (#12784)

* Issue #12622: Fix creating Prefix using VLAN without site

* Issue #12622: Fix importing Prefix using VLAN without site

This commit also adds tests to verify the import changes implemented
in this commit.

* Issue #12622: Cleanup code to filter allowed VLANs on a prefix import

* Closes #12622: Switch to VLAN selector dialog when creating Prefix

netbox/ipam/forms/model_forms.py

@@ -211,10 +211,8 @@ class PrefixForm(TenancyForm, NetBoxModelForm):
     vlan = DynamicModelChoiceField(
         queryset=VLAN.objects.all(),
         required=False,
+        selector=True,
         label=_('VLAN'),
-        query_params={
-            'site_id': '$site',
-        }
     )
     role = DynamicModelChoiceField(
         queryset=Role.objects.all(),