Add List-Id header to send emails like GitHub to help GMail filtering work

Problem to solve

It's hard to filter GitLab emails per group/projects on GMail.

Proposal

Add a List-Id header to emails like GMail does: For example, for this project, it would be:

List-ID: gitlab-org/gitlab-ce <gitlab-ce.gitlab-org.gitlab.com>

What does success look like, and how can we measure that?

The "List-Id" should be in the header and GMail can filter on it.

More

GitHub also adds "List-Archive" with the URI of the project, which can be nice.