Mastodon API: Fix lists leaking private posts
Our previous list visibility resolver grabbed posts if either follower
collection of the user in a list who is followed is in to
or if
follower collection of the user in a list was in cc
. This not only
missed unlisted posts but also lead to leaking private posts when
fix_explicit_addressing
mistakingly started putting follower collections
to cc
(also fixed in this MR).
Reported by @kurisu@iscute.moe via a DM