Move the responsibility for opening the sidebar when there is a draft to
edit from the guest into the sidebar's `FrameSyncService`.

The initial motivation for this was to make the logic in the
`beforeAnnotationCreated` event handler in the `Sidebar` class work if
the annotation is created in a non-host guest frame. The plan was to
make the sidebar send an `openSidebar` request with custom options that
would trigger the logic that used to be in the `beforeAnnotationCreated`
event handler.

However it appears that focusing the sidebar frame is no longer necessary. In my
testing the input field in the sidebar was successfully focused regardless
of whether `iframe.contentWindow.focus()` was called or not. Therefore I
have just removed the handler.

Even though the original need no longer exists, moving the logic still
seems sensible because logically it is the sidebar application that
knows it is awaiting user input after a new draft is created. It also means that
if we do find a need to re-introduce the frame-focusing logic that used
to be in the `beforeAnnotationCreated` handler, we can do so in a way
that works with annotations created in non-host frames.