This patch is the third part of subscription confirmation feature.
Support to send email to subscriber if confirmation is needed.
Change-Id: I230f5c7fbc9d19554bbcf34ce9b2f3b14230321b
Implements: blueprint subscription-confirmation-support
(cherry picked from commit 4778f708fa9ec86b4137eefd63d07c40ad24296e)
f196f9a8a9