Make CentralAuth login more silent

Change the redirect-through-central-domain to a 302 instead of a
JavaScript post (I'm told Safari 5 users aren't a concern) and make
the iframes load silently on the pageview after the login rather
than on a login success screen.

This adds the feature with a feature flag, which can be removed in a
future revision.
Rewrite p-personal on successful central login check

It has been decided we should do this, and hope that the discrepancies
between this and a real login don't cause users any confusion.