Require auth for /explore and /help when public visibility is restricted

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Related to (extension of) gitlab-ce#3508

When public level is restricted on an instance, groups and projects are not public. This can lead to confusion when the /explore page is empty and it can also be undesirable since all other aspects of the GitLab instance are private. We should require authentication on the /explore and /help endpoints in this case.

cc/ @DouweM What do you think?