mw.ViewPageTarget: Hide the welcome dialog via a preference if possible

For logged-in users, we can a preference instead of a cookie. This way it is
also preserved between browsers and when cookies are cleared.

Keep using cookies for logged-out users, except if the beta welcome dialog
has been suppressed using the one-off GET parameter 'vehidebetadialog'.

Bug: 55551
Change-Id: Ica9e5a92841fec003ce4a21d740a9bc6ff3da9c7
Merge "mw.ViewPageTarget: Hide the welcome dialog via a preference if possible"