reconsider grouping of review comments

[annthurium]

we got some feedback from a user, via twitter:

https://twitter.com/flaximus_prime/status/1118210262446821376

"When there are many comments in a review, and some are resolved, it’s hard for me to scan that list and pick out those not resolved. They’re interspersed and it just doesn’t work for my eyes/brain. I’d love a way to hide the resolved ones or group them."

We tried to validate some hypotheses about comment ordering / grouping during our initial user research, but the results were inconclusive. It could be worth revisiting comment grouping/ordering, now that we have something shipped.

[vanessayuenn]

Closed via #2117