handle PDFs hosted via WordPress on other file storage systems

When WordPress is configured to host its Media Library in a different environment than the local file system (eg, AWS S3), this plugin doesn't launch the Hypothesis client with PDFs stored in such a
"foreign" Media Library using via.hypothes.is. This may have to do with cross-domain issues.

To recreate:
* Use a WP instance configured to store its Media Library in another place (eg, an AWS S3 plugin like https://deliciousbrains.com/wp-offload-s3/).
* Upload a PDF to the Media Library and verify it is stored in an alternate location (eg, AWS S3).
* Navigate to the PDF location and verify that it launches without via.hypothes.is.
* You may be able to test the issue by forcing the PDF to be stored locally again and launch it to see if it then launches with via.hypothes.is.