LLP: fix templating problem for login page

Since the login page is using the view created
by django.contrib.auth, we need to ensure that
the "site_name" context variable is set.

Change-Id: I5e5160ffd54383718b9942d06871611cb038e818
Signed-off-by: Kelley Spoon <kelley.spoon@linaro.org>
Reviewed-on: https://review.linaro.org/c/infrastructure/linaro-license-protection/+/40739