Display web header and footer system messages in email header and footer

Currently we have a feature to display a custom message on every page in GitLab, in the header and the footer. See https://docs.gitlab.com/ee/customization/system_header_and_footer_messages.html.

This issue will take that same content, and display it in every email, optionally, for the header and footer in the email body too.

There doesn't need to be any styling in the email for this content. In the original feature, there is styling customizations. These are not required and should not be carried over to the email content. The email body should simply display the same header and footer entered.

Also see this relevant change for the original feature: https://gitlab.com/gitlab-org/gitlab-ce/issues/55057.

We need to communicate that the text content will be in emails, but that the colors would not be reflected.

Proposal