FEATURE: Enables support for dark mode emails (#16520)

This PR enables custom email dark mode styles by default that were added here.

There is currently poor support for dark mode queries in mail clients. The main beneficiary of these changes will be Apple Mail and Outlook.

Enjoy the darkness 🕶️

spec/integration/email_style_spec.rb

@@ -72,7 +72,7 @@ describe EmailStyle do
         SiteSetting.default_email_in_reply_to = true
 
         expect(mail_html.scan('<h1 style="color: red;">FOR YOU</h1>').count).to eq(1)
-        matches = mail_html.match(/<div style="([^"]+)">#{post.raw}/)
+        matches = mail_html.match(/<div style="([^"]+)" dm=\"body\">#{post.raw}/)
         expect(matches[1]).to include('color: #FAB;') # custom
         expect(matches[1]).to include('padding-top:5px;') # div.body
       end