Support different meta og:img per site

[mcglincy]

This may be: add a field to SystemResource and use it in meta_tags_component.rb.

I'll let John verify before I create a PR.

[jdbann]

Yeah, that could work. You'd need to add something like has_one_attached :share_image to the System model as well.

Also to make sure that the various sites can cache this effectively, we should use rails_storage_proxy_url(Current.system&.share_image) to generate the image_src attribute in MetaTagsComponent.

Without it, Rails generates signed URLs which load directly from the S3 bucket. That's fine for users and saves on server load, but signing it means that the URL is different each time and would keep invalidating the caches for the social platforms.