Redirect zero-rated users more nicely

Related patch (merge-deploy both):

I86963587f441423e072711c6da08b0ace730c005

In the event the user hits zero.wikipedia.org/ (webroot), but
zerodot isn't in the zero-rating list, send the user to mdot.
Furthermore, if there's a good language homepage match, ideally
zero-rated, send the user to that language homepage match.

We're trialing this on the zero-rated experience. In the future
if this goes into mdot Wikipedia at large, we'll need to ensure
that traffic that isn't eligible for zero-rating doesn't take
zero-rating rules into account, although the language detection
logic would otherwise work the same.

See following URL:
https://www.mediawiki.org/wiki/Wikipedia_Zero/
Accept-Language_Aware_Redirects

The Vary header in mediawiki-config will be updated in a
separate patch to have Accept-Language.

Change-Id: I31d93509afc1f5620c8f60d8d1052f63735ae0e8
Merge "Redirect zero-rated users more nicely"