Group artifacts by project in notifications

[yrodiere]

Sometimes, a project contains a great deal of artifacts that all follow (approximately) the same versioning scheme. For instance, Spring: https://www.artifact-listener.org/project/spring/

It would be great if we could just follow the whole project, so that when we receive a notification, the individual artifacts that changed would be "grouped" under the same project. That way we could inspect the details if we want to, but we would easily see that it's just a bunch of artifacts from the same project.

On a sidenote, it would also be nice to be able to follow a project directly, instead of following artifacts independently.