broaden MQL defn to include people who register for our events

Created by: sqs

This broadens the definition of MQL to include people who attend a webinar. I think we were already operating with this definition, and the handbook was just wrong/outdated. (If MQL intentionally excludes this, that is fine, although I'm curious why.)

(Should it be even broader, including people who merely RSVP yes to a webinar but don't actually end up attending?)