Update favicon from admin panel

[allinfinite]

For instances utilizing managed hosting services, this would really help.

[Ragzouken]

Would also appreciate this option.

From this comment mastodon#7396 (comment) it is clear mainline mastodon does not want to add this capability, but I think hometown (or at least the ideas presented in build your own social) agrees with the idea that instances are individual entities with their own identities that want to be distinguished. As someone running a small private instance along those lines, I do not "benefit from the trust established through brand recognition" and the users will not be confused about this.