[bug:ui] Select2 autofilter look is inconsistent with other filters

[nemesifier]

Describe the bug

The auto filter using select2 widget has a different look than the rest of the filters.

I think the look should be consistent.

I don't think this change will cause problems to change forms, but let's double check

Steps To Reproduce Steps to reproduce the behavior:

1. Go to any admin list page which has the select2 filter, eg: device list in openwisp-controller
2. Click on the select2 filter and notice the difference with other filters

Expected behavior

Consistent look.