When injecting the client into a guest frame, proxy any custom asset locations
specified via `.js-hypothesis-config` script tags in the host frame.
These are used by the browser extension and needed for the VitalSource
integration to work in that context.

We could probably avoid copying the `sidebarAppUrl` and `notebookAppUrl`
settings for guest-only frames. This will require changes to the boot
script and possibly the annotator entry point.