[SensioGeneratorBundle] Rename skeleton template files to twig extension

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none
| License       | MIT
| Doc PR        | none

In our system, we make a pre-commit syntax validation based on extension of files. But these files are twig templates not phps.