Remove or fix referer header for file downloader

[G1org1owo]

This was introduced all the way back in ab493a0, but I have noticed how it creates some problems with yande.re in particular. When making a request to an image such as this with an explicit Referer: header which does not belong to the domain yande.re, the host returns the post html page instead of the actual image.

This causes szurubooru not to recognize the received file and throw an "Unhandled file type: 'application/octet-stream'" error.

I have not opened a pull request as I believe this should be discussed first, but I see no easy way to dynamically retrieve the correct referrer and therefore think the header should be scrapped altogether. Moreover, it is currently set as if the request was coming from the target URL itself, which I'm pretty sure isn't the correct usage of the header anyways.

Is there something that would break, were it to be removed?