[web] Display correctly color splotch, even when username is long

Test Plan: Log in as a user with long username and check if color splotch in chat thread list is displayed correctly

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D886