[web] Move ThreadSettingsModal and related components to new modals/threads/settings directory

Summary:
web/modals/threads was getting crowded by ThreadSettingsModal-related files... which are going to continue to grow in number.

Created a new settings sub-directory in threads to consolidate the relevant files. Typically prefer to avoid heavily nested directory structure, but I think it makes sense here.

Test Plan:
Imports were "fixed" by my IDE/flow/eslint.

Also opened ThreadSettingsModal in browser and clicked around making sure things appeared/worked as expected.

Reviewers: def-au1t, palys-swm, benschac

Reviewed By: palys-swm, benschac

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4043