[web] Use callback version of set state for setQueuedChanges

Summary:
Addresses feedback from @palys-swm here: https://phabricator.ashoat.com/D3827?id=11833#inline-23376

Unfortunately couldn't name the argument queuedChanges because that's already defined in the scope and led to eslint warnings, so had to go with prevQueuedChanges which I'm not a huge fan of and imo makes things a bit less readable.

Test Plan:

1. Open ThreadSettingsModal
2. Make some changes (name, description, color)
3. Log the "queued changes"
4. Hit save
5. Ensure that the "queued changes" are made as expected

Reviewers: def-au1t, palys-swm, ashoat, benschac

Reviewed By: ashoat

Subscribers: Adrian, karol-bisztyga, yayabosh, palys-swm

Differential Revision: https://phabricator.ashoat.com/D3890