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/lib/email/styles_spec.rb

@@ -157,10 +157,6 @@ describe Email::Styles do
   end
 
   context "dark mode emails" do
-    before do
-      SiteSetting.dark_mode_emails_active = true
-    end
-
     it "adds dark_mode_styles when site setting active" do
       frag = html_fragment('<div class="body">test</div>')
       styler = Email::Styles.new(frag)