Migrate to relative CSS units (WCAG 2.1 AA)

[jonnystening]

We must move away from px units and size typography and spacing using relative (rem) units. This is in order to respect the user's browser ‘font size’ setting, configurable within the browser settings (options generally provided as Very small, Small, Medium (recommended), Large, Very large).

Note: This relative sizing update will not affect the Grid module (or our breakpoints), which will remain in px units.

A number of changes were made to the GUI Framework modules in order to accommodation the new WCAG 2.1 AA specification. Details of further updates can be found at #433.