Fix relative redirect handling
Fix relative redirect handling

The download URLs for PDFs uploaded to Blackboard return a *relative*
redirect (with the leading `https://blackboard.hypothes.is` missing). We
apparently haven't encountered relative redirects before because they
don't work: Via 3's `/proxy/static` endpoint (the endpoint that is
served by NGINX directly) fails to re-add the missing
`https://blackboard.hypothes.is` prefix before redirecting and ends up
redirecting the browser to
`https://via3.hypothes.is/proxy/static//bbcswebdav/courses/developer-test-course-1/uploaded-directly.pdf?VxJ...%3D`
(note the `//` in `/static//bbcswebdav` where the missing
`https://blackboard.hypothes.is` should be).

Fix the static endpoint to insert the missing
`https://blackboard.hypothes.is` prefix.

Testing:

To test this with Blackboard Files:

1. Check out this WIP branch of the LMS app:
   https://github.com/hypothesis/lms/pull/1965

2. Log in to https://blackboard.hypothes.is/ using the `administrator`
   account (it's in 1Password)

3. Go to Developer Test Course 1 and launch Blackboard Files Assignment.
Choose Select PDF from Blackboard then click Authorize. **It doesn't
actually work**, but the authorization popup window should have caused a
temporary Blackboard Files download URL to be printed to the terminal.
You can paste the URL into http://localhost:9083/

To test that things still work with non-Blackboard files:

* An HTTP PDF file: http://localhost:9083/route?url=http%3A%2F%2Fwww.orimi.com%2Fpdf-test.pdf

* An HTTPS PDF file: http://localhost:9083/route?url=https%3A%2F%2Fwww.w3.org%2FWAI%2FER%2Ftests%2Fxhtml%2Ftestfiles%2Fresources%2Fpdf%2Fdummy.pdf

* A PDF file behind a redirect: http://localhost:9083/route?url=https%3A%2F%2Fh.readthedocs.io%2F_%2Fdownloads%2Fen%2Flatest%2Fpdf%2F

Test assignments: https://hypothesis.instructure.com/courses/125/assignments

Known issues:

* This only works for hosts that support https. `https` is hard-coded
  when inserting the `https://$proxy_host` prefix in
  `direct_proxy.conf`.  This is because I couldn't find an NGINX
  variable for the scheme (http or https) of the URL being proxied.

  Having this work for https hosts only is better than it not working at
  all.

hypothesis/via,hypothesis/via,hypothesis/via