Use correct MIME types for raw submission source

[quantum5]

441 introduced the raw submission source view, but it always sends text/plain, which is not exactly appropriate. Perhaps future browsers will be able to syntax highlight based on MIME type or something. Who knows?

[quantum5]

This is probably a bad idea: serving V8JS submissions as application/javascript or even text/javascript is most likely going to be abused to inject JavaScript from a CSP-trusted domain.