[web] Refactor - remove instances of useMemo + classNames combo

Summary:
Currently in the codebase, we have callsites where we're using the useMemo hook with the classNames library. This is unnecessary as useMemo returns a memoized value, which in this case is just a string and hence safe to run on every render.
https://linear.app/comm/issue/ENG-2287/[web]-do-not-wrap-classnames-in-reactusememo

Test Plan:

• checked for all occurences by git grep 'React.useMemo(() => classNames' -- *.js as well as a VS Code search.
• started and interacted with the web app to ensure nothing became broken along the way

Reviewers: atul

Reviewed By: atul

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5697