Restore previous HTML form in a different location

This commit restores the previous HTML page that is currently used by
the STS wizard at a different location. This allows a single instance
to serve both the new UI and the (now) STS-specific form.

The LegacyStsController can be used to serve that endpoint and is not
auto configured. app.groovy has been updated to explicitely import that
bean.

The tests infrastructure has been abstracted a bit so that both pages
are actually tested with a set of common tests.

Fixes gh-33