This diff cleans up the general + privacy tabs in the thread settings modal to follow the conventions of the new modal api. Since both the general + privacy tabs had very similar logic for their save button, to cleanup these tabs I introduced a new ThreadSettingsSaveButton which factored out all the shared logic of the save button and passed this new save button component as the primaryButton for these two components
Linear task: https://linear.app/comm/issue/ENG-5943/extendmodify-the-modal-props-api-to-follow-new-modal-designs
Depends on D11033