Integrated Alerting: Sending email using Gmail fails

Original report:
I configured the Communication to send email using gmail and I'm getting this error in the log:
msg="Notify attempt failed, will retry later" attempts=13 err="send MAIL command: 530 5.7.0 Must issue a STARTTLS command
first. c16sm3151674wrw.89 - gsmtp"

User Impact:
Users can't use Gmail or another SMTP service on the nonstandard port to send emails in Alerting.
Steps to reproduce:
Configure SMTP settings using 587 port
Emulate some Alert in PMM and try to use the email channel to receive notification about it

Actual result:
Notify attempt failed, will retry later" attempts=13 err="send MAIL command: 530 5.7.0 Must issue a STARTTLS command
first. c16sm3151674wrw.89 - gsmtp"
Expected result:
Email is sent

Solution:
Add an additional flag to require the SMTP TLS in the modal form where we configure the SMTP settings.