Fix notes email with group-level notification email
What does this MR do?
When we introduced group-level notification emails, there was a bug in how we routed comment emails: Noteables do not have a group directly (unless they are Epics) - they are on a project which may have a group.
Fixing this was pretty small, but did involve more spec changes than I'd have liked.
Does this MR meet the acceptance criteria?
Conformity
Performance and testing
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Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/63355.