Adds a button to the navbar that toggles the sidebar in the reader view. The
state is persisted in localStorage, so it survives reloads and navigation.
The button is also hidden on mobile, where FreshRSS already provides its own control for showing/hiding the sidebar.
Download this repo into a folder named xExtension-ToggleSidebar, upload it to
your ./extensions directory and enable it in the extension panel in FreshRSS.