* Add email_header_and_footer_enabled flag to appearances table
* Set email_header_and_footer_enabled default value to false
* Add checkbox to appearance to toggle show header and footer in emails
* Add email_header_and_footer_enabled to allowed params in controller
* Add header and footer messages to the html and text email layouts
* Remove the color styling for emails header and footer
* Add empty_mailer layout for emails without layout,
to have the header and footer applied
The References header is an ordered list of mails, and some clients
only look at the last mail in the list to determine to which mail the
mail is in reply to.
Therefore, to be a less obtrusive as possible, put the fallback
address first.
Some info about the reply key lookup in the References header:
https://docs.gitlab.com/ee/administration/reply_by_email.html#how-it-works
Sean McGivern