Adds introduction tab with updated design

[davidkrantz]

This is the initial PR for the introduction. It basically contains the same code as we had last year, but with a few modifications. Additional tweaks are probably still needed before we can say that #240 is resolved.

• Adds introduction tab
• Adds updated introduction images (stone buttons same as last year, but will be changed later)
• Updates color scheme (may need some additional tweaking at some places)

[Lullebullelukas]

Let the summerproog begin!