Add image processing and generate blurhash for server thumbnail

[Gargron]

The 1200x630px size or even the format has not been enforced before. Turns out you cannot trust server admins not to upload gigantic TIFF images into the server thumbnail setting even if the hint says that the 1200x630px size is recommended. To be honest, there is nothing surprising or even outrageous about it, the surprising part is that 2017-me believed otherwise. Now we will generate web-friendly variants specifically for the thumbnail as well as a blurhash value to handle loading placeholders.

Remove separate server hero setting. Not a lot of sense in keeping the hero and thumbnail as separate settings when I would wager they should be the same in most cases. Reason for keeping thumbnail over hero is that the hero setting has never been exposed in the API. Through the API, the thumbnail appears on the servers list on joinmastodon.org, in the official mobile apps, and possibly other third-party apps, so it has more reach than the hero. Related: #19345

In GET /api/v2/instance, the thumbnail property becomes:

{
  "url": "...",
  "blurhash": "...",
  "versions": {
    "@1x": "...",
    "@2x": "...",
  }
}

[ykzts]

mastodon/app/views/about/more.html.haml

Line 12 in 4705387

= image_tag @instance_presenter.hero&.file&.url || @instance_presenter.thumbnail&.file&.url || asset_pack_path('media/images/preview.png'), alt: @instance_presenter.title, class: 'parallax'

I think this line also needs to be fixed (but will not be needed in #19345).

[Gargron]

I think this line also needs to be fixed (but will not be needed in #19345).

Yes, I figured maybe I can skip it here to avoid merge conflicts.