Reply-by-email did not pick up /unsubscribe quick action

Summary

I sent an reply email to unsubscribe myself from an issue: https://gitlab.com/gitlab-org/gitlab-ee/issues/3764#note_59330751

But the system did not parse out the quick action, and just posted the comment.

Steps to reproduce

1. Open a notification email
2. Reply that email with /unsubscribe

Example Project

https://gitlab.com/gitlab-org/gitlab-ee/issues/3764#note_59330751

What is the current bug behavior?

It posts a comment with:

/unsubscribe

What is the expected correct behavior?

It unsubscribes you from the issue, and no comment is posted.

Output of checks

This bug happens on GitLab.com

Possible fixes

Ask @DouweM