Disallow repeats (retweets) of private notices

Disallow repeats (retweets) of private notices

We disallow repeating a notice (or whatever) if the scope of the
notice is too private. So, only notices that are public scope
(available to everyone in the world) or site scope (available to
everyone on the site) can be repeated.

Enforce this rule at a low level in Notice.php, and in the API,
commands, and Web UI. Repeat button doesn't appear on tightly-scoped
notices in the Web UI.