[web] Change the Apps tab to be a + button

Summary:
Issue: https://linear.app/comm/issue/ENG-2792/change-the-apps-tab-to-be-a

Test Plan: Run web app, checked that the button displays correctly, and pressing it navigates to Apps screen

Reviewers: michal, kuba, kamil

Reviewed By: kamil

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6733