Use committer email to check mailmapped x509 commits

What

Update Draft: Use author email to check for mailmapped... (!190822 - closed) to use committer email to check mailmapped x509 commits.

Why

Currently (behind ff) if a committers email is mailmapped we check that the new email address is one of those present in the signatures x509 certificate.

But we do not check if the email is also verified to the user who made the original commit.

To solve this Draft: Use author email to check for mailmapped... (!190822 - closed) adds an author_email column to the x509_commit_signatures table to store and track the un mailmapped email address which pushed the commit. But based on this discussion we should actually be check committer_email here instead.