theme: optimization for huge screen sizes

[McShelby]

On huge screens (width > 1500px (dpr-adjusted)), the layout gets difficult to capture and read. We can do better by leveraging the additional space.

https://css-tricks.com/optimizing-large-scale-displays/

To adjust for arbitrary font-size settings, the menu was refactored again. The follwing things are fixed with this:

• resizes and positions correctly according to html elements font-size
• IE11 compatiblity is now given
• a11y: expander is now correctly given a focus outline
• chevron is now coded into html instead of a css content value, which makes it easier to change (was an issue in the past)
• the visited checkmark now breaks correctly on long lines and hides behind the text if necessary
• some more css classes were introduced to make further extensions (eg. nested shortcut links) easier

[McShelby]

For now, we made sizing proportional to the initial font size.

By just giving a new font-size to the `html´ element, the whole site will adapet.

html{
  font-size: 24px;
}