The current inter-frame communication doesn't work if an annotatable
(guest) iframe is from a different origin than the host frame (see
https://github.com/hypothesis/client/pull/3611#issue-696947005). This
will be fixed in a more comprehensive overhaul of the inter-face
communication (see https://github.com/hypothesis/client/issues/3533).

Meanwhile, I add a scenario into the local dev server where the
annotatable iframe is from an origin different than the host frame. For
this, I needed to spawn an additional dev server at port 3002):

```
[11:32:50] Dev web server started at http://localhost:3000/
[11:32:50] Dev web server started at http://localhost:3002/
```

Close #3629