Epics don't set useful mail headers (like X-GitLab-Project-Path)

Summary

I just realized mail notifications for epics don't set any mail headers that could be used to sort them in a useful way (e.g. by project / group)

Steps to reproduce

• Subscribe to an epic, for example one at https://gitlab.com/groups/gitlab-org/-/epics
• Check the mail header

What is the current bug behavior?

Excerpt from the mail header (filtered by X-GitLab-*)

X-GitLab-Discussion-ID: 5ae9b28f36fa9bfb6dce2dc3d32dcddc8a857818
X-GitLab-Epic-ID: 2424
X-GitLab-Epic-IID: 633
X-GitLab-Reply-Key: [...]

-> nothing that could be used to filter by project / group / etc.

What is the expected correct behavior?

Should set additional header fields like issue notifications do.

Example headers (filtered by X-GitLab-*):

X-GitLab-Project: GitLab.com Support Tracker
X-GitLab-Project-Id: 63904
X-GitLab-Project-Path: gitlab-com/support-forum
X-GitLab-Issue-ID: 9454974
X-GitLab-Issue-IID: 3097
X-GitLab-Reply-Key: [...]