[web] introduce thread settings utils

Summary:
Move diff, I created this file to house all the business logic for the thread settings modal into separate hooks. We need to do this because we need to use this logic outside of this component and use it in a "primaryButton" prop that we will pass to the modal, and I thought having this logic preemptively in hooks would make the code more readable when we introduce the necessary primaryButton

Linear task: https://linear.app/comm/issue/ENG-5943/extendmodify-the-modal-props-api-to-follow-new-modal-designs

Depends on D11030

Test Plan:
flow + confirmed that the thread settings modal still works as expected

general:

privacy:

Delete:

error:

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11031