[web] Remove LeftLayoutAside

Summary:
Since D6344 the LeftLayoutAside is obsolete. I'm removing it, and putting the CommunityPicker directly in the App component.
This also calls for removing the left-layout-aside.css, which I'll do in the next diff, as it requires some code to be moved somewhere else.

Test Plan: Run the web app, check that the sidebar displayes correctly.

Reviewers: przemek, michal, bartek, tomek

Reviewed By: bartek, tomek

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6336