Improve About Page

Closes #1376 (closed)

Left todos:

• on mobile, banner should take the full width of the screen
• on mobile, each feature takes 2 lines instead of 1
• on mobile, statistics are presented in a single column, while they should make use of the width of the screen
• Moderators (there is a todo in the sources) (not possible)
• Add a changelog snippet
• logged in users are presented with the signup box