Feature Request: ingest "already seen" classification metadata flag

[lcjohnso]

Context: a series of individuals with high classification counts was investigated and identified to be unwanted behavior. One distinguishing feature of the high-rate classification behavior being unwanted: many of the subjects were flagged as "already seen" and fallback subjects were each classified many times.

Feature Request: add ingest of "already seen" metadata as binary flag on classification_events table as way to identify and track possible abusive behavior.

[yuenmichelle1]

Closed with #80