Change email address on homepage to use an environmental variable

[rgossiaux]

https://github.com/cardinalitypuzzles/cardboard/blob/0bbe1c7173c239baf5b949f2a16554ebcf41616c/cardboard/templates/home.html#L32

Other teams deploying this won't want to use cardinalitypuzzles@ (we don't want them using that either!).