403 forbidden page & graphic

[rlskoeser]

@fontnerd could you make a hippo variant for a forbidden page?

Once the hippo is created we just need a 403.html template parallel to the 404 one here: https://github.com/chnm/serendipomatic/blob/master/templates/404.html

Amy, I think you haven't messed with the templates much, so feel free to assign the issue to someone else when the hippo is ready. :-)

This isn't too much of an issue right now (I only know of one way to trigger a 403 and I did it accidentally), but I think it will be more possible to crop up occasionally once we have implemented the full Zotero login functionality that we're targeting for November.