[web] Refactor ChatTabs to use Tabs component

Summary:
Used introduced Tabs component in ChatTabs, replacing existing solution. Chat borders design now matches Figma design and elements have onHover effect. The behavior of panel should remain the same.

Before:

After:

Test Plan: Run web app and try to switch Between Focus and Background tabs. It should work as before.

Reviewers: palys-swm, benschac, atul, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, Adrian, atul, karol-bisztyga, benschac

Differential Revision: https://phabricator.ashoat.com/D3373