Introduce bootstrap's spinner as throbber

Problem/Motivation

As discussed in #3024839: Update to Bootstrap v4.3.1 → we want to introduce the (growing, my preference) spinner as throbber (aka ajax-progress).

This is not always wanted/needed so we introduce a new theme setting, which allows to use it "Never" (off) or "Frontend only" (on).

We introduce a third option for using it "always", as the theme can only inject/override libraries if active this to be done in bs_lib.

Regarding the design, I thought of using the primary color for the background to make the throbber look familiar. And I added a border and box-shadow in the currentColor (which is used by bootstrap for the background) to ensure good visibility everywhere.

Dependencies

Needs the new component from here #3047335: Provide new component for spinner / ajax-loading → .
When releasing it needs to be done together with bs_lib.