[web] Use new Tabs.Container instead of existing Tabs component

Summary:
Use the new Tabs.Container component in ThreadSettingsModal to get things closer visually.

There's still a good amount of styling to do here... this is just the first step

Depends on D3844

Test Plan:
Here's what it looks like (for now.. will re-style momentarily):

Reviewers: def-au1t, palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, Adrian, karol-bisztyga, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3845