Manage Email Notifications with Queues

[poppastring]

When adding comments, we have the options to send emails. These emails should be managed via queues not synchronously in the web request. :(

[clemensv]

I agree. The notification handlers for entries use the thread pool queue for that.

Also see #671. We should also raise events for comments and allow for the email notifications (etc.) to be handled externally, e.g. in Logic Apps/Power Automate if someone would like to use that.