Add app version to JS and CSS

This leads to the regeneration of the hash in case a single application is updated. - This does not touch the Assetic Pipeline because I don't know how it works.

Fixes https://github.com/owncloud/core/issues/11374

@karlitschek Shall we backport this?
```php
\OC_App::getAppVersions()
```

No need to do a loop