Make headers always visible

[mlqn]

Description of the bug

The headers (Header.tsx and AltinnHeader.tsx) does not remain visible when users scroll down the page. This can lead to a bad user experience, as users may have difficulty navigating the site. To improve user interaction and overall usability, we should make the headers always visible when scrolling by using something like: position: sticky; top: 0;

This bug was reported by @standeren.

They should also remains above other elements, using something like: z-index: 999;

Steps To Reproduce

Dashboard

https://github.com/Altinn/altinn-studio/assets/24462611/fd789e64-c635-4951-afc1-08cd368d1967

Overview

https://github.com/Altinn/altinn-studio/assets/24462611/c3d5a5b5-359e-469c-a1a5-4c35138fd790

New Overview

https://github.com/Altinn/altinn-studio/assets/24462611/6c66a625-da19-4a05-b23b-54731625612d

Contact

https://github.com/Altinn/altinn-studio/assets/24462611/038ad445-6748-463e-b803-1e308f7d8914

404

Deploy

Additional Information

No response