[web] introduce copy username functionality to user profiles

Summary:
As part of the scope for user profiles, we want to be able to copy the user's username onto your clipboard. This diff adds this functionality into the copy username div already on the user profile

https://linear.app/comm/issue/ENG-4963/copy-username-functionality

Depends on D9410

Test Plan:
Please see the demo video below

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9411