Introduce the UI components library

Bootstrap offers a nice set of UI components that can be reused when Moodle core or Moodle plugins needs to output UI elements like navigation, alerts, dropdowns and much more

See:

http://getbootstrap.com/docs/4.0/components

When these basic elements can be called from the core renderer it will make the overall Moodle UI more consistent. The amount of core CSS can be reduced and updating to newer versions of Bootstrap would take a lot less time.

A lot of the groundwork for this has already been written in theme_boost and in core_renderers and can be reused. 

This would also replace core elements like AMD powered modals and drop-downs, and ultimately forms, tables and any other Moodle UI element.