Don't show reblog feature if post's blog is private.
Merge branch 'develop' of github.com:wordpress-mobile/WordPress-iOS into issues/1782-disable-reblogging-if-private
Let service fail if reblogging a private post.

Adds test.
Merge pull request #1828 from wordpress-mobile/issues/1782-disable-reblogging-if-private

Issues/1782 disable reblogging if private