Make Title::makeTitleSafe() not need user variant

Make Title::makeTitleSafe() not need user variant

In trying to avoid hitting RequestContext::getLanguage() from a call to
User::idFromName(), I05aacd30 made it no longer safe for user input.

A closer analysis of the call stack involved reveals that
Title::makeTitleSafe() is constructing a prefixed dbkey using the
localized name for NS_USER, and then Language::getNsIndex() is needing
to get the variant in order to handle that localized name. But if we use
the canonical name for NS_USER, Language::getNsIndex() short-circuits
and skips the problematic code path.

And it turns out that it doesn't actually matter which prefix
Title::makeTitleSafe() uses, since the prefix doesn't make it anywhere
into the resulting Title object. So let's revert I05aacd30 and Ibeef0409
and just do that instead.

Change-Id: Ib902573996c69d1e77527cc7b2faf4e7fa5d3daf