New style Shadow dom

[timea-solid]

• This branch implements ShadowDOM with CSS custom properties.
  But also HTML5 and basic ARIA for accessibility.
• Each Card has a dedicated style file in the style folder and code for loading styles in the Shadow DOM.
• global.css contains the so-called CSS custom properties, which define colors and such -> this file would theoretically be moved, possibly to mashlib or overwritten.
• Details about what can be customized as of now is: https://github.com/SolidOS/profile-pane/blob/shadowDom/CSS_CUSTOM_PROPERTIES.md

[timea-solid]

We decided not to implement shadow dom. see: SolidOS/solidos#229