FIX: Improve tags in email subjects and add filter headers (#19760)

This commit does a couple of things:

1. Changes the limit of tags to include a subject for a
   notification email to the `max_tags_per_topic` setting
   instead of the arbitrary 3 limit
2. Adds both an X-Discourse-Tags and X-Discourse-Category
   custom header to outbound emails containing the tags
   and category from the subject, so people on mail clients
   that allow advanced filtering (i.e. not Gmail) can filter
   mail by tags and category, which is useful for mailing
   list mode users

c.f. https://meta.discourse.org/t/headers-for-email-notifications-so-that-gmail-users-can-filter-on-tags/249982/17

lib/email/message_builder.rb

@@ -161,6 +161,11 @@ module Email
       result['X-Discourse-Post-Id']  = @opts[:post_id].to_s  if @opts[:post_id]
       result['X-Discourse-Topic-Id'] = @opts[:topic_id].to_s if @opts[:topic_id]
 
+      # at this point these have been filtered by the recipient's guardian for visibility,
+      # see UserNotifications#send_notification_email
+      result['X-Discourse-Tags'] = @template_args[:show_tags_in_subject] if @opts[:show_tags_in_subject]
+      result['X-Discourse-Category'] = @template_args[:show_category_in_subject] if @opts[:show_category_in_subject]
+
       # please, don't send us automatic responses...
       result['X-Auto-Response-Suppress'] = 'All'