* 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
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