Cross reference branches with Experimenter API

[mikewilli]

We've had instances where a changed/deleted branch has some data in it (likely from QA or other testing before launch) and this leads to an errant branch in the results, which is at best annoying and at worst can break the UI*. We should see if we can cross-reference the branches in enrollment telemetry with the ones in the Experimenter API response for each experiment.

*separately we'll try to make the UI more robust to these issues, but then that leaves us with the best-case scenario where it's still annoying to see errant data in the results

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