[web] Rename AppSwitcher to NavigationPanel

Summary:
We're introducing a new menu (https://www.figma.com/file/a1nkbWgbgjRlrOY9LVurTz/Comm-%2F-Desktop-app?node-id=1272%3A88750) that looks exactly like AppSwitcher. In order to do that, we're going to create a reusable component that renders such a menu. To make review process easier, I'm starting with renaming the concrete component to what will be a reusable component, so that it's obvious what will be kept and what will be moved to the concrete implementation.
Depends on D3337

Test Plan: Use AppSwitcher to change between chat and calendar and check if it works correctly.

Reviewers: benschac, atul, def-au1t

Reviewed By: benschac, atul

Subscribers: ashoat, Adrian, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D3338