Make possible for self-hosted instances to set their own favicon and link preview

Instance on foodsharing.se would like to use it’s own favicon, as it is Karrot’s now.

There’s also the link preview showing info about Karrot. That might create some confusion when sharing the link to our instance in platforms that give a preview. Related to #2617 - Support link previews - karrot/karrot-frontend - Codeberg.org