Wait for sidebar to setup before attempting annot scroll
Wait for sidebar to setup before attempting annot scroll

- the `openSidebar` method wasn't actually returning a `Promise`, although all invoking code treated it as if it did
- should fix #618
- regressed in e6693cfdc86309b7bd1c1e5249eee1f656cec861

WorldBrain/WebMemex,WorldBrain/WebMemex