Adding this should allow us to only notify users that haven't been
notified before.
Necessary because the change_added event does not include a preData that
allowed us to diff requestedApprovers based on the event alone.
Also added a index on this column, since we're going to be using it to
filter.
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Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>
