OpenID feature on translatewiki UI

In case you wonder why it hasn't happened yet: given the bad experiences I've had with the practically unmaintained Liquid Threads, I don't want to take another (even more) complex extension on my plate here.

We will be often the first ones to find issues, and often it falls on our shoulders to also fix those issues, because nobody else gives a damn until the same problems are on their face, maybe years later if at all.

It will happen when I'm convinced that OpenID works reliably and there is someone to support it for the foreseeable future.