Hide private replies properly when the right settings are set.

I thought the initial code would work, but it turns out that
private replies to users we aren't following can have
in_reply_to_status_id null *and* empty attentions.

So, reuse the code that checks whether a post starts with an @
to hide the reply if it's not directed at us or a user we're
following.
3 jobs for hide-private-replies in 3 minutes and 45 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Lint
passed #6996
lint

00:01:09

 
  Build
passed #6997
build

00:01:21

 
  Test
passed #6998
test

00:01:13