Skip Special:CentralAutoLogin/start for JS anons

The call to Special:CentralAutoLogin/start from
ext.centralauth.centralautologin.js isn't strictly necessary, we can
just pass in (via ResourceLoader) the central wiki URL for
Special:CentralAutoLogin/checkLoggedIn, append some parameters, and go
there directly. And apparently that would be a big win cache-wise, so
let's do it.

Bug: 54195
Change-Id: I2817a1b1b01f0719b460b3a745010884b26b1774
Merge "Skip Special:CentralAutoLogin/start for JS anons"