Extend subscription and notification service

[uklotzde]

Currently all e-mails (signup, subscription, password reset) are dispatched to a local sendmail instance. This has various restrictions and drawbacks:

• Poor performance, i.e. one external process invocation per e-Mail
• The synchronous invocation doesn't scale
• Sending e-mails doesn't work within the Docker container and is disabled

The current implementation should be replaced by dispatching the sending of e-mails to a service provider.

Overview: https://konghq.com/blog/list-of-10-email-apis/

• MailGun
• SendGrid
• Mandrill
• mailjet
• MailChimp
• ...

The implementation should define an abstract interfaces that allow switching from one provider to another if required and doesn't introduce any dependencies on the actual API into the use cases.

Related issues:

• 166 Decouple post-processing from requests

• 172 Email-format issue

• 44 Send notifications as daily summary

• 45 Add link to unsubscribe in notification mail

• 95 Changes in email-texts

• 68 Support email templates

ToDo's

• [ ] Design overall architecture
• [ ] Create a components diagram
• [ ] Implement it
• [x] https://github.com/kartevonmorgen/openfairdb/pull/259

[uklotzde]

All the subscriptions and notifications should be handled by an external service. After the introduction of GET /entries/recently-added in #198 this is now possible.

[wellemut]

E-Mail API gateway is done, https://github.com/kartevonmorgen/openfairdb/pull/259 but not in use yet. Our server still sends mails directly. Needs to change.

[wellemut]

@magdaddy What is the state of this issue at the moment?