Prevent sub-threshold GRBs to be preferred external event over threshold GRBs

Currently, sub-threshold GRBs can be preferred over external events. We need to check certain conditions to prevent those from threshold GRBs. https://git.ligo.org/emfollow/gwcelery/-/blob/81d6af856846d7a79a390085beb604f12b09bd19/gwcelery/tasks/raven.py#L301-308

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