[web] cleanup thread settings general + privacy tabs

Summary:
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

Test Plan:
Please see the demo videos below

privacy tab:

general tab:

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11034