Feature: Include participants at the bottom of PM emails (#5797)

* Feature: Include participants at the bottom of PM emails

... as undecorated links.

https://meta.discourse.org/t/email-notification-recipients-unclear-when-pm-is-sent-to-multiple-users/26934/13?u=featheredtoast

Fix: missing translation for PM mentions

* display membership count as `group (count)`

lib/email/message_builder.rb

@@ -96,6 +96,13 @@ module Email
         html_override.gsub!("%{respond_instructions}", "")
       end
 
+      if @template_args[:participants].present?
+        participants = PrettyText.cook(@template_args[:participants], sanitize: false).html_safe
+        html_override.gsub!("%{participants}", participants)
+      else
+        html_override.gsub!("%{participants}", "")
+      end
+
       styled = Email::Styles.new(html_override, @opts)
       styled.format_basic
       if style = @opts[:style]
@@ -112,6 +119,12 @@ module Email
       body = @opts[:body]
       body = I18n.t("#{@opts[:template]}.text_body_template", template_args).dup if @opts[:template]
 
+      if @template_args[:participants].present?
+        body << "\n"
+        body << @template_args[:participants]
+        body << "\n"
+      end
+
       if @template_args[:unsubscribe_instructions].present?
         body << "\n"
         body << @template_args[:unsubscribe_instructions]

lib/email/styles.rb

@@ -97,6 +97,7 @@ module Email
       style('.lightbox-wrapper .meta', 'display: none')
       correct_first_body_margin
       correct_footer_style
+      style('div.undecorated-link-footer a', "font-weight: normal;")
       reset_tables
       onebox_styles
       plugin_styles