batches: Default to oldest global external service

Created by: eseliger

We had an issue in one of our instances where a new global external service was configured and then batch changes broke by that. If we said we default to the oldest external service instead, this would never change unintentionally. It is sort of a breaking change, but one for a deprecated feature that never really was documented how it works. So my personal feeling is that this is alright. While at it, I also removed user-added external services, using those didn't make sense in the first place as long as we don't use them in the ForUser authenticator.

Test plan

Has an extensive test suite.