views.py: check X_FORWARDED_FOR if set for whitelisted IP

If run behind a proxy, LLP will always see a request as coming
from the proxy's IP address.  This change updates the IP check
to use the X_FORWARDED_FOR header to check the client's IP for
whitelisting internal hosts.

Change-Id: I1f220301d7ac8658141407d930f255fc0d4f2e13
Reviewed-on: https://review.linaro.org/c/infrastructure/linaro-license-protection/+/31458
Reviewed-by: Benjamin Copeland <ben.copeland@linaro.org>