[web] Convert thread-settings-modal.css selectors to use snake_case instead of dash-case

Summary: We discussed this a bit ago in the Comm Dev Team chat, noop

Test Plan:
careful reading, things continue to look the same at a glance

Went through every occurrence of "css" in ThreadSettingsModal and changed the selector names to underscore from dash.

The two occurrences of CSS selectors in ThreadSettingsModal that still have dashes are current-tab and modal-radio-selector. These don't exist in the CSS yet and are left over from before (where it was also broken).

Reviewers: ashoat, palys-swm, benschac

Reviewed By: ashoat

Subscribers: Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3622