Track performance degradations back to merge requests that caused them

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It would be nice to run sitespeed.io benchmarks against staging after an MR gets merged when we know for sure which pages are affected by it. That way we could do a quick performance regression testing and point to a root cause of the degradation.

The main problems here are:

1. Knowing when to run the benchmarks (do we run them with some label present or always?)
2. Identifying which pages to run against

When a degradation is detected a bot should ping all the responsible individuals for the page and for the MR.