prerelease: Run pa11y tests when there is no `pa11y.mustache` template.

pa11y tests should run if there is a demo with name `pa11y` which
compiles to `pa11y.html` in the demos directory. The demo may or
may not be based on a template `pa11y.mustache`.

This commit also removes references to "custom config" (it would
have been awkward to commit separately). Custom config is a way
of configuring components outside of origami.json. This is not
documented anywhere in spec v1, moving to spec v2 is a good time
to drop suppport.