[portal] Skip auth when admin endpoints are exposed on an internal port.
[portal] Skip auth when admin endpoints are exposed on an internal port.

Will be used in k8s environment, at least initially. This approach assumes we
don't expose any really critical information/abilities via /admin/portal pages.

If we ever do, we should figure out a better authentication story (perhaps piggy
back on k8s auth somehow). Few hours of research didn't find anything easily
implementable.

R=5de1a387cc854ff3d04fab3d66d4e77a58511771@chromium.org, ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org, tandrii@chromium.org
BUG=959427

Change-Id: I99d5cd73786e86bd093ccf5d74d8d7c56e1c3e07
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1621749
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Reviewed-by: Jao-ke Chin-Lee <5de1a387cc854ff3d04fab3d66d4e77a58511771@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go