[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