Annotation of binary default images does not submit the annotated image

[eyelidlessness]

All of the same notes as #1034

Additional context

I believe the probable solution on the enketo-core side (and this may be the only remaining work following #1041 🤞 ) is to populate the annotation field's canvas on load when a binary default is present, rather than referencing it separately by URL.

[eyelidlessness]

As mentioned in the Express issue, this was user error not a bug. But it did help me catch a regression in enketo/enketo-core#960!