[web] Adjust SubchannelsModal size to match designs

Summary:
Changed SubchannelsModal content size to match design in Figma.

Test Plan: Open SubchannelsModal from thread actions menu

Reviewers: palys-swm, atul, benschac

Reviewed By: atul

Subscribers: ashoat, palys-swm, Adrian, atul, karol-bisztyga, benschac, yayabosh

Differential Revision: https://phabricator.ashoat.com/D3672