Set User-Agent header in proxied media requests to match currently spoofed device

[chowder]

Fixes #465

I've been running this patch for a few days, and it works. Credits to @bennettmsherman for actually discovering the fix.

I've contemplated adding the entire header set from Oauth, as is done for non-media requests here, but I ended up getting blocked instead.

It really is just the user-agent header that needs to be given.

[dmachaj]

I have an instance running on a raspberry pi and it has degraded to the point where it is almost unusable. I built this commit locally and can also confirm that it repairs things back to previous levels.

I think it would be nice to merge this so that the Docker image could be updated to include it. I don't want to run a debug build long-term. I'd rather go back to Docker.

Edit: I had good luck with Dockerfile.alpine and building my own image locally with this fix in place. Aside from taking 20+ minutes to build on a raspberry pi 4 it worked great.

[sigaloid]

Thanks!