Importance Classification: Add a feature to the classifier to be able to distinguish between user interaction and automated tagging, e.g. user manually assigned or unassigned importance to an email

Follow-up to https://github.com/nextcloud/mail/issues/3968

The importance classifier could be further improved by adding a feature to distinguish between important tags set by the automation vs. manually by the user. This would also prevent positive feedback loops.