Add List-Id to notification emails

What does this MR do?

We recently switched to GitLab and I discovered that I couldn't filter GitLab notification emails by project easily in GMail. What GitHub does is that they "synthetize" a List-Id header from the project's name and fullpath. This patch should do the same thing.

What are the relevant issue numbers?

Implements #53493 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines

Notes

I'm not sure if I need a changelog entry for this? Also, I'm not sure if the Ruby coding style is correct.