[web] Fix ThreadSettingsGeneralTab save button disabled prop

Summary:
Fixed the save button in ThreadSettingsGeneralTab which was previously not staying disabled if the user had selected another color and then changed their selection back to the current thread color (making no other changes in the thread settings) (see before/after video in Test Plan).

Related Linear issue with full context here.

Test Plan:
Tested on Chrome/Safari and works as expected.

Before:

After:

Reviewers: atul, palys-swm, ashoat

Reviewed By: atul, ashoat

Subscribers: ashoat, palys-swm, Adrian

Differential Revision: https://phabricator.ashoat.com/D3980