Actually disable disabled filter-groups

In STCOM-166, I added a notion of "disabled" filter-groups. As a placeholder implementation, I made disabled groups have a grey background, as shown in the UX prototype at https://share.proto.io/245SJH/ – the relevant code is in https://github.com/folio-org/stripes-components/pull/163/files and the horrible style line is https://github.com/folio-org/stripes-components/blob/20ac319e9f7f0405044ee45f0020eb9d3a6c9b5c/lib/FilterGroups/FilterGroups.js#L42

JohnC, can you please fix my hideous implementation of what "disabled" means? Refer to the UX prototype for how it ought to look, and make is so that the checkboxes correctly display their values but cannot be toggled. Thanks!