More my-account cleanup/consistency

More my-account cleanup/consistency

For consistency, make new sub-pages (payments history, preferences) load
as a separate page in the same path (prefs -> prefs_notify) instead of
either an ?expand= option or a sub-path page (prefs -> prefs/notify).

Let the my-account base page load the generic base page as a wrapper so
that my-account page are not required to load both and (for me, anyway)
make the inheritance more obvious.