Revise signup page

[brandstetterm]

Description

The current version of our signup page doesn't fit anymore to our new landing page and the new template/sessions page that's going to be implemented soon. Therefore, it's time to think about how we could revise our signup page.

[SelinaBuff]

I would recommend to use our "new" background (light/dark), which we use also for our LP or our future template page. We should stick to the planned template page in terms of structure anyway.