restyle all user notification emails to use a custom html template

lib/email/message_builder.rb

@@ -6,7 +6,9 @@ module Email
     def build_email(*builder_args)
       builder = Email::MessageBuilder.new(*builder_args)
       headers(builder.header_args) if builder.header_args.present?
-      mail(builder.build_args)
+      mail(builder.build_args).tap { |message|
+        message.html_part = builder.html_part if message
+      }
     end
   end
 
@@ -22,11 +24,12 @@ module Email
                         user_preferences_url: "#{Discourse.base_url}/user_preferences" }.merge!(@opts)
 
       if @template_args[:url].present?
-        if allow_reply_by_email?
-          @template_args[:respond_instructions] = I18n.t('user_notifications.reply_by_email', @template_args)
-        else
-          @template_args[:respond_instructions] = I18n.t('user_notifications.visit_link_to_respond', @template_args)
-        end
+        @template_args[:respond_instructions] =
+          if allow_reply_by_email?
+            I18n.t('user_notifications.reply_by_email', @template_args)
+          else
+            I18n.t('user_notifications.visit_link_to_respond', @template_args)
+          end
       end
     end
 
@@ -36,6 +39,27 @@ module Email
       subject
     end
 
+    def html_part
+      return unless html_override = @opts[:html_override]
+      if @opts[:add_unsubscribe_link]
+        html_override  << "<br>".html_safe
+
+        if response_instructions = @template_args[:respond_instructions]
+          html_override << PrettyText.cook(response_instructions).html_safe
+        end
+
+        html_override << PrettyText.cook(I18n.t('unsubscribe_link', template_args)).html_safe
+      end
+
+      styled = Email::Styles.new(html_override)
+      styled.format_basic
+
+      Mail::Part.new do
+        content_type 'text/html; charset=UTF-8'
+        body styled.to_html
+      end
+    end
+
     def body
       body = @opts[:body]
       body = I18n.t("#{@opts[:template]}.text_body_template", template_args) if @opts[:template]