Avoid an extra network round trip when proxying PDFs

[seanh]

When a URL like https://via.hypothes.is/https://example.com/foo.pdf calls the proxy() view, the view:

1. Determines whether https://example.com/foo.pdf is HTML or PDF (by making a GET request to https://example.com/foo.pdf and looking at the Content-Type)
2. Renders an <iframe> whose src is either https://via.hypothes.is/pdf?url=https://example.com/foo.pdf (which goes to the view_pdf() view) or https://viahtml.hypothes.is/proxy/https://example.com/foo.pdf depending on whether https://example.com/foo.pdf is a PDF or not

This causes the browser to make an extra network round trip to get the <iframe>'s src URL.

In the case of PDFs this extra network round trip could be avoided: the proxy() view could just return the PDF.js response (as would be returned by the view_pdf() view) directly.

In the case of HTML the proxy() view does still need to return an <iframe> since HTML is handled by a separate app.