Use url_for to allow running CTFd on a subpath

[tyhdefu]

Thanks for your work on this plugin.

Due to the hardcoded paths in index.html, the plugin doesn't work when run on a subpath (e.g. example.com/ctfd).
This is probably only done when testing, and probably is never a problem on a production deployment, but if you're interested in the fix then merge it.

I haven't managed to explicitly test this without a subpath, but I think this is the "flask" way of doing things and follows how the rest of the ctfd templates work so I highly doubt there would be an issue.