Inject tiddlers for social media preview metadata

The next release of TiddlyWiki will include a
$:/core/templates/social-metadata tiddler that will produce metadata
used by social media link previews.

Provide some tiddler content to make that work for TW5 sites on
Tiddlyhost.

See TiddlyWiki/TiddlyWiki5#8441

rails/lib/settings.rb

@@ -82,6 +82,10 @@ def self.subdomain_site_url(site_name)
     ActionDispatch::Http::URL.full_url_for(Settings.url_defaults.merge(subdomain: site_name))
   end
 
+  def self.site_thumbnail_url(site_name)
+    "#{subdomain_site_url(site_name)}/thumb.png"
+  end
+
   def self.tiddlyspot_site_host(site_name)
     "#{site_name}.#{Settings.tiddlyspot_host}"
   end

rails/lib/th_file.rb

@@ -94,6 +94,15 @@ def apply_tw5_mods(site_name, for_download:, local_core:, use_put_saver:, is_log
 
       # Provide a way for TiddlyWikis to detect when they're able to be saved
       '$:/status/IsLoggedIn' => status_is_logged_in(is_logged_in:, for_download:),
+
+      # Used in `$:/core/templates/social-metadata` for social media preview metadata
+      # It seems like it would be unexpected for these to be included in a downloaded
+      # file, so set them to an empty string in that case.
+      '$:/SiteUrl' => (for_download ? '' : Settings.subdomain_site_url(site_name)),
+      '$:/SiteDomain' => (for_download ? '' : Settings.subdomain_site_host(site_name)),
+      # Todo maybe: Don't overwrite this if the user customized it. Creating this as a
+      # shadow tiddler would be a nice way to achieve that but I don't how to.
+      '$:/SiteThumbnailUrl' => (for_download ? '' : Settings.site_thumbnail_url(site_name)),
     })
 
     # Since every save uploads the entire TiddlyWiki I want to discourage

rails/lib/tw_file.rb

@@ -167,6 +167,7 @@ def to_html
   def write_tiddlers(tiddlers, shadow: false)
     if json_store?
       # Assume we're using TW 5.2 and later
+      # Todo: Not sure how to create a shadow tiddler like this
       append_json_store(tiddlers)
 
     else