Add a template attribute for filtering whether to send the email or not

this field must evaluate to TRUE to send the email

and it should default to the TRUE string

and it should affect the status saved to the sending report

No files or tests are named. Start by locating the Handlebars template rendering and the code that sends emails and writes the sending report; trace where template attributes are evaluated. Done means the new attribute defaults to the TRUE string, prevents sending when it does not evaluate to TRUE, and produces the corresponding report status.

Written by the indexing model from the issue text.