Run Stagecraft module generation when app builds

This runs the tool to generate module files when the app is built
in either development or production, as well as running it before
the tests start.
Remove page-per-thing stubs and add ignore in git

They're generated as part of the build process.
Merge pull request #301 from alphagov/linter-autogenerate-page-per-thing

Autogenerate module stubs in linter