require a store in the Theme

This means removing the feature for preloading the Theme with
already-created templates, but I think that's a marginal feature at best
(and a simple subclass could bring the feature back).

The error message is far more useful than the ability to add some
templates beforehand.
set up Statocles website

The Statocles website is built with Statocles! There is some issue with
having a non-root site though...

Fixes #113