* 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
Adds `#build_notification_recipients` to `NotificationRecipientService`
that returns the `NotificationRecipient` objects in order to be able to
access the new attribute `reason`.
This new attribute is used in the different notifier methods in order to
add the reason as a header: `X-GitLab-NotificationReason`.
Only the reason with the most priority gets sent.
