Add a global, site-wide setting for issue help text

There's currently support for per-project help text when users submit issues. However, it'd be nice to also have a global setting to do site-wide messages.

The current use-case is on d.o, we'd like to add some global help text that's something like: "If you're about to report a security issue, you're doing it wrong -- please submit it directly to the drupal security team and keep it confidential, instead of publicly reporting it here" (with better wording and links to the info they need, of course).