Abstract out common code of Users and Items top-level settings

Oh, I suppose I shouldn't say that I did it "nicely"

Nicely done and documented: https://github.com/folio-org/stripes-components/blob/master/lib/Settings/readme.md

The work in stripes-components is in this commit: https://github.com/folio-org/stripes-components/commit/28e305b5d7e9b6e27b9b5698447a27bcbfa12425

I've also released a new v0.7.0 of stripes-components that includes this; and upgraded the dependency in both the Users and Items modules; and changed them to rely on the component.