Load messages from translatable on-wiki page

Break out suggestSiteLink() into smaller methods

Minor visual adjustments, including making this a 'large' MessageDialog.

In a future commit we can switch to using an OOUI ProcessDialog, which will eliminate most if not all the custom styling we're doing, as well as surface any API errors we're currently ignoring.

Add oojs-ui as a dev dependency to assist IDEs.