Custom instance thumbnail generates broken URL

lib/pleroma/web/mastodon_api/views/instance_view.ex:

thumbnail: Pleroma.Web.base_url() <> Keyword.get(instance, :instance_thumbnail),

iex(pleroma@10.27.3.200)4> Pleroma.Web.base_url() <> Keyword.get(instance, :instance_thumbnail) "https://safe.millennial.spacehttps://safe.millennial.space/media/ddbb3dc7b1d63183c6ffa123606c6c17211f56ee6c345ae54f0e4266b734c227.jpg"

When you upload a custom instance thumbnail via AdminFE the URL to the image file is no longer relative, so this breaks the image. This is kind of required because your media storage may be on another domain.

I know there have been requests in the past to make all media links relative for Tor users so they never make requests through the clearnet, but to fully support that I think we'd have to have some kind of Tor mode setting that disables things like custom base URLs or somehow force everything through MediaProxy over Tor.