Custom email campaigns

[S-Haime]

Is your feature request related to a problem? Please describe.
Other platforms, such as Wiley, have options for opt-in email campaigns for readers. It has been requested / suggested Janeway develop such a functionality. The example given:
"I’d like to have a separate opt-in option (or opt-in e-mail list) to be able to send out 4 messages per year that issues have been completed. "

Describe alternatives you've considered
The reader role + notification for publication have been considered, but a 'digest' people can opt-in to would meet these needs better.

Additional context
See FD #9384

Was also discussed in stand-up and agreed to be a good idea.

[S-Haime]

@mauromsl could you just give this a quick glance and see if I missed anything, based on: https://janeway.freshdesk.com/a/tickets/9384

If so, feel free to comment - otherwise ,just give it a thumbs-up :)