Prevent modifying entity type of attributes

There is no good reason to allow modifying entity type of
CollectionType and SeriesType attributes.  The only use case would
be to fix careless mistake before the edited attribute gets being
used.  This is safer and simpler to disallow changing it altogether
rather than check related entities and parent/children attributes.

lib/MusicBrainz/Server/Controller/Admin/Attributes.pm

@@ -93,6 +93,8 @@ sub edit : Chained('attribute_base') Args(1) RequireAuth(account_admin) {
     my $form = $c->form( form => $form_name, init_object => $attr );
 
     if ($c->form_posted && $form->process( params => $c->req->params )) {
+        $form->field("entity_type")->value( $attr->entity_type );
+
         $c->model('MB')->with_transaction(sub {
             $c->model($model)->update($id, { map { $_->name => $_->value } $form->edit_fields });
         });

root/admin/attributes/form.tt

@@ -36,6 +36,13 @@
     [% ELSE %]
         [%~ IF model == "CollectionType" || model == "SeriesType" ~%]
             [% form_row_select(r, 'entity_type', add_colon(l('Entity type'))) %]
+            [%~ IF r.form.field('entity_type').value.size ~%]
+                <script>//<![CDATA[
+                    $(function () {
+                        $('#id-attr\\.entity_type').prop('disabled', true);
+                    });
+                //]]></script>
+            [%~ END ~%]
         [%~ END ~%]
 
         [% form_row_select(r, 'parent_id', add_colon(l('Parent'))) %]