The initial un-hiding and opening of the sidebar was previously
triggered by a `panelReady` event emitted by the Guest in the host frame
when it connects.

As part of eliminating the need for the host frame to also be a guest,
change the initial sidebar open to use the `Sidebar.ready` promise
instead, which is resolved when the sidebar applicaiton sends a
`hypothesisSidebarReady` notice to the `Sidebar` app.

Part of https://github.com/hypothesis/client/issues/3798