[web] Limit username length in account menu

Test Plan:
Log in as a user with long username and check if username is displayed correctly.
Log in as a user with short username and check if username is displayed as it was before the changes.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D885