This commit updates the RSS post content to use email formatting. Many
plugins are using the `reduce_cooked` method to format content that is
not displayed outside of Discourse application. Using email formatting
also strips the secure media and various other things that is only meant
for Discourse client side application.
RSS spec says the author tag should be an email address, forcing us to put in a junk no-reply@example.com email. Instead, we should use dc:creator, which allows us to use any name we want for the user.
