Add 302 Redirect for /code/<project> to https://github.com/18F/<project>

I have not tested this on `staging`, looking to @konklone to deploy and test this out. Expected outcome should be to provide a 302 redirect from `https://18f.gsa.gov/code/18f.gsa.gov` to `https://github.com/18F/18f.gsa.gov`.
No, 403 is appropriate. The `redirect` in the `rewrite` line should produce a 302, the 403 will return if there is some other error ("server refuses to respond to request").