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
c4ea158656