allow unauthenticated viewers to view campaigns

Created by: sqs

Fixes https://github.com/sourcegraph/sourcegraph/issues/11501.

Unlike #11528, this lets unauthenticated viewers actually view the campaigns list and detail pages as well. Currently there are no instances where this would occur (because the only instance with unauthenticated viewers is Sourcegraph.com, and that is already special-cased), but I am hacking on such an instance now (https://sourcegraph.slack.com/archives/C0W2E592M/p1592357208354100?thread_ts=1592279188.347200&cid=C0W2E592M) and would appreciate this slight extra flexibility!