feat: user dropdown menu for account, settings, extensions, etc.

Created by: sqs

Previously, the top nav had too many links. They took up a lot of width and added clutter. Now, there is a user dropdown menu. It also includes a sign-out button, which is a fairly frequent user request.

If there is no authenticated user, it still shows a "Sign in" button and some of the links. Authenticated users no longer see "Saved searches", "Discussions", and the theme switcher (because it soon will persist to user settings, not localStorage).

This PR does not need to update the CHANGELOG