Implement ability to send emails

This issue is about the basic implementation of a mail service in the RSD.

User story (example)

This is an example for a user story. The implementation of this specific user story is not part of this issue:

As a software/project/organisation maintainer, I would like to be notified if something relevant happened with my software/project/organisation.

Relevant events

Selection of potential events where the user would benefit from being notified via email.

As a software maintainer

• my software has been unpublished by someone who is not a software maintainer, see !128 (closed)
• my software has recently been cited (automatic mention scraping not supported yet)

As an organisation maintainer

• someone referenced my organisation in a software or project

Implementation

This feature is relevant upstream.

Q: Do we have to ask the user for permission so we can send them emails?

related

• project-management#47
• use cases for email notifications (upstream #893)
• send email notifications (upstream #894)
• %License consultation