fixes core#2318: fix cancel button when sending an individual email

[MegaphoneJon]

Overview

You can't cancel a non-CiviMail email. Screencast is present on core#2318 and written replication steps on core#2815

[civibot]

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[eileenmcnaughton]

MOP - treating @MegaphoneJon as the 'reviewer' here as @sluc23 proposed the fix