Create new accessible single-select element

See MDL-70075 and related issues for some of the history, but we currently use an HTMLSelect element for our Single Select element type and the behaviour for an HTMLSelect is non-normalised between different browsers/OS combinations, and most of those combinations do not allow us to provide a WCAG-compliant solution. We should create a new single-select element which makes use of the Bootstrap Dropdown component, and which complies with WCAG 3.2.2.