Drop filtered messages if the filter is “irreversible”

Drop filtered messages if the filter is “irreversible”

… instead of adding them to the timelines and then not showing them.
This fixes timelines showing new items when the only new items are
“irreversibly” filtered toots. This may be an edge case in Mastodon/glitch-soc,
but it is not in Pleroma, which does no filtering server-side whatsoever.