Email footer isn't consistent between mentioned, assigned, and general activity

Summary

When emails are sent because someone has been assigned the email footer shows:

You're receiving this email because you have been assigned an item on gitlab.com.

When an email is sent when you are mentioned the email footer shows:

You're receiving this email because of your account on gitlab.com. 

Steps to reproduce

1. Assign an issue or merge request to a user
2. Mention a user in an issue or merge request

What is the expected correct behavior?

Footer should show the appropriate reasons for the email:

  def notification_reason_text(reason)
    string = case reason
             when NotificationReason::OWN_ACTIVITY
               'of your activity'
             when NotificationReason::ASSIGNED
               'you have been assigned an item'
             when NotificationReason::MENTIONED
               'you have been mentioned'
             else
               'of your account'
             end