[Epic] New landing page

Following the discussion at https://governance.funkwhale.audio/d/aDqFmMuT/customizable-landing-page-for-instances, we're going to redesign our anonymous landing page.

The main goal is to make this page more valuable by:

• Providing quick access to login and signup forms
• Displayable customizable content (banner, description) so that each pod can express their own identity
• Display/Link to important info such as pod rules, contact info, etc.
• Explain in a concise and accurate way what Funkwhale is

Current look

Initial wireframe for the redesign

Tasks

• Work on a final wireframe
• Add a banner setting (!893 (merged))
• List and implement the various settings we need to offer:
  • Server description
  • Server name
  • Server rules, extended info (we'll use the "Long Description" field for this purpose)
  • Terms of service (!!894 (merged))
  • Custom CSS (#879 (closed) and !818 (merged))
  • Anything else?
• Add those settings to the Nodeinfo endpoint
• Implement the new UI

Target release is %0.20.