Move sidebar closing to a button in the navigation bar

The sidebar currently has two ways to close:

- Click the "x" in the upper right corner of the sidebar panel
- Click the active icon in the sidebar switcher

The first one is pretty obvious, but the second one is a bit confusing.

## Proposed Alternative

We could replace both options with a single "toggle sidebar" option in the navigation bar. This is a pattern many apps follow, so should be familiar to users. Removing the close button will give us a new open place to put common actions for that sidebar.

### Mockup

The issue does not name files or tests; start by locating the sidebar, sidebar switcher, and navigation bar components in the React frontend. Confirm the existing two close interactions, then verify that the navigation-bar toggle is the single closing option and that the former close button is removed.

Written by the indexing model from the issue text.