Provide an option to replace text with icons

Problem/motivation

For optimum presentation, it is sometimes optimal to show icons instead of text links. This could apply to the individual calendar links, and also the add to calendar link that launches the modal.

Proposed resolution

Create an SVG sprite containing logos for Outlook, Google, and Apple, as well as a calendar icon. Create a library that would use CSS to show the icons instead of the equivalent text. Add a "use icons" setting to this module that if enabled will attach the new library in the twig template.