Prevent proxying audio content or block it

[jon-betts]

After we have worked out if we can block/redirect by video content type (https://github.com/hypothesis/via3/issues/286), we may want to try and see if we can extend this to audio types.

For an example see: https://qa-viahtml3.hypothes.is/proxy/https://www.vox.com/pages/podcasts