[web] Fix strange visual abberations in ChatMessageList

Summary:
Something changed in Chrome recently and now min-height isn't working properly. There are a host of strange visual effects resulting from this.

The solution here is unideal in that it uses a calc and has us copy-pasted the number 62px, but I've already spent way more time on this than I should've and I can't find a better solution.

Test Plan: Make sure visual abberations are gone

Reviewers: atul, palys-swm

Reviewed By: atul

Subscribers: KatPo, zrebcu411, Adrian, subnub

Differential Revision: https://phabricator.ashoat.com/D893