[META] Track interesting Bootstrap features

Problem/Motivation

While it is relatively easy to update bs_bootstrap theme to new Bootstrap releases, fare more work requires to implement new features that are coming with major releases. This issue will track all interesting features that we should look into more details and maybe implement:

• Spinner loading component added in 4.2, #3047339: Introduce bootstrap's spinner as throbber
• Toast component for displaying notifications, added in 4.2
• New iOS style switch for checkboxes, added in 4.2
• New responsive .row-cols classes, added in 4.4
• New add() and subtract() functions for avoiding errors and zero values from CSS's built in calc feature, added in 4.4
• Added new .navbar-nav-scroll class for scrolling expanded navbar contents on mobile devices, added in 4.6

There is also old #3024839: Update to Bootstrap v4.3.1 with additional discussion after 4.3.1 update. @TODO - check it for more ideas.