[web] Restyle ThreadSettingsDeleteTab

Summary:
There aren't any designs for this in Figma, so tried to get close to what we have in ThreadSettingsGeneralTab. There's definitely room to improve the design here, this was strictly a first pass.

Depends on D4018

Test Plan:
Before (start of stack):

After: (after this diff):

Reviewers: def-au1t, palys-swm, benschac

Reviewed By: palys-swm, benschac

Subscribers: ashoat, Adrian, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4019