[web] Disable Save button after settings are saved in ThreadSettingsGeneralTab

Summary: Relevant Linear issue here. Also see @tomek's comment on D4416. This issue arose after we decided to leave ThreadSettingsGeneralTab open (Linear issue) after the user had clicked the Save button, since the Save button remained enabled even if there were no changes to be made.

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

Reviewers: tomek, jacek, atul

Reviewed By: tomek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D4931