centralauth-account-exists-reset should give link to a wiki where the account exists

In this patch I just add a link to Special:CentralAuth; the user will then have
to click one of the wiki names and then access Special:PasswordReset there.
I have no idea on how to dynamically generate an URL to one such wiki in here.

Bug: 37219
Change-Id: I502f227ab573303b3499f92bc731b88c8ec310b7
Merge "centralauth-account-exists-reset should give link to a wiki where the account exists"