Support notifications configuration

**Feature request**
It would be great to have notifications for validation.

**Use case**
Many users need this functionality for production deployments.

**Implementation idea**
Add a new config option `enable_notifications` with default `false`.

No files or tests are named. Start by locating the configuration handling and validation flow, then determine what notification behavior and delivery mechanism are required; the issue is done only when enable_notifications defaults to false and the agreed validation notifications work for production deployments.

Written by the indexing model from the issue text.