[web] Adjust MembersModal size to match designs

Summary:
Changed MembersModal content size in order to make the whole modal size exactly the same as in design.

Test Plan: Open Members modal 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/D3671