R1:6a996d76fd1a

Remove "feed.public" and sort out feed policies

Summary:
Ref T6817. Fixes T8731. On the old `secure` host, `feed.public` was set to `true`. I didn't bring the option over, which caused the secondary issue in T8731.

Specifically, when `feed.public` is off, a logged-out user looking at feed can't see //any// stories, so they query all of feed until they hit the time limit.

To fix this immediately, just use the most open policy, which is basically equivalent but always correct.

To fix this more thoroughly…