Capture and Compare Simultaneously

Presently, capturing and comparing are two separate steps. First you capture your full list of URLs (multiple times). Then you specify which two distinct runs you want to compare against. But it seems like some of this could be done at the same time.

Upon initiating a capture, if we had an optional "Compare with Run" option, we could specify a particular run to compare against and compare while we capture.

An example, of how this may actually come in handy is configuring a job to always compare against the most recent run (if it has one).

This would mean for every capture run, a comparison run would be completed as well. This would probably be a prerequisite for #2922937: Don't save duplicate captures.

Another use case would be for development(PR Builder) environments. Upon a successful build we could automate that WPA crawl those dev environments and always compare them with the latest run against the production job. That use case would probably require #2975917: Drush 9 support be completed first.