Improve the "registermoochtips" hub string

Raised in chat, right now, when a site is not registered with Moodle, trying to set moodle.net (formerly "Mooch") as hub, shows this string at the top of the page, together with a <<register>> button:

$string['registermoochtips'] = 'In order to register with Moodle.net, your site must be registered with Moodle.org.';

MD suggested it could be, instead, something clearer and more informative like:

Register your site with Moodle to get security alerts and access to Moodle.net, our course sharing platform.

Ciao