[web] Leave ThreadSettingsGeneralTab open after settings are saved

Summary:
Relevant Linear issue here. Leaves ThreadSettingsGeneralTab open after hitting the "Save" button. This ensures that the user can make any more changes if they'd like to even after clicking "Save" without having to reopen the modal manually.

Will add a nicer transition to the Save button in a subsequent diff. See Linear issue for that here.

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

Reviewers: atul, palys-swm, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, Adrian

Differential Revision: https://phab.comm.dev/D4416