Refine design of all pages and components

[felixerdy]

Think about how we could improve the design of the pages and components.

• cleaning
• heading / description
• maybe images / icons --> https://undraw.co/illustrations
• show "create survey" button only when logged in

[georgi-s]

I think I need to do something more about heading. But I am not sure. We can also talk about the icons tomorrow.