Allow new type of theming to co-exist with old.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| Refs tickets  |
| License       | MIT
| Doc PR        | 

The better way of theming will be to use template inheritance
features of Twig. As such a controller simply includes a base
template (which can be overriden by the theme system).

This is a simple hack which prevents the old theme from rendering
if module was found using routing (which only new styles can).