Disable the email sending menu entries when sandboxed on OS X

The way we send email doesn't work anyway in a sandboxed app on OS X. If we
want to offer the functionality, we need to implement it in an OS X -specific
way, perhaps using the Scripting Target functionality of Mail.app.

What we need, to be really "native" in this regards on OS X, would be
to have a "share" functionality that would in addition to sharing
through email offer AirDrop, Facebook, Twitter etc. Like Apple's own
apps. Quite possibly there are system APIs that automate much of
that. But they probably require your app to be more "native" in other
ways, too, like using NSDocument etc. Later...

Change-Id: Id1424377b7d493d4ec696f3242dae55a2a67dfc7