[web] Copy/paste user-settings-modal.css into thread-settings-modal.css

Summary:
noop

Next diffs:

1. Move the styles used in thread-settings-modal.react from style.css to thread-settings-modal.css.
2. "Consume" thread-settings-modal.css from thread-settings-modal.react instead of style.css.
3. Remove extraneous selectors from thread-settings-modal.css that aren't used in thread-settings-modal.react.
4. Fill in styles that are "used" by thread-settings-modal.react that don't actually exist anywhere in the repo (eg current-tab). Probably by looking through the git history to see what current-tab used to be.
5. Style the ThreadSettingsModal component so it looks decent.

Test Plan: NA, literally copy/paste and the pasted code isn't even used anywhere

Reviewers: def-au1t, palys-swm, benschac, ashoat

Reviewed By: ashoat

Subscribers: Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3589