Missing API Owners gate for "Write up motivation" stage

[jrobbins]

According to launching-features, feature owners are supposed to send an "Intent to deprecate" during the "Write up motivation" stage and get 3 LGTMs for it. I think that we completely missed this when we added all the cross functional review gates.

Also, in detect_intent.py, the "Intent to deprecate" subject line is detected, and it is currently ignored because of the missing gate. Once we add the gate, we should check that the detected intent is actually processed rather than being ignored like "intent to prototype" threads.