chore: Convert PX values to REM

This MR converts pixel values to REM. It also replaces some literals defined in components (i.e. $gl-datepicker-width) with values based on the $grid-size variable.

The function to convert px to REM is a port of an implementation CSSLab CSSLab.

Using REMs instead of pixels make our components more accessible. They resize according to the browser default font-size.

Instead of defining REM values directly, we are using a conversion function to improve development experience. The design system specs use pixels as unit of measure. Using a conversion function saves us the time of translating the design system specs to REM in our heads.

Closes #398 (closed)