LandingPage: treat as HTML field, prefer short_description

Summary

Both instance.description and instance.short_description are considered HTML fields by Mastodon: https://docs.joinmastodon.org/methods/instance/

Recently No Agenda launched with Soapbox and it shows the issue: https://fe.noagendasocial.com/

Screenshots (if appropriate):

Fixes this:

So it looks like this: