[web] introduce MessageReactionsListItem

Summary:
This diff simply factors out the logic of rendering the message reactions list item into it's own component. We want to do this so that we can use usePushUserProfileModal inside this component

Part of https://linear.app/comm/issue/ENG-5244/make-user-profile-accessible-from-message-reactions-modal

Depends on D9484

Test Plan: Confirmed that the message reactions modal still looks and behaves the same

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9485