Implement file reference wiget

apps/files/composer/composer/autoload_classmap.php

apps/files/composer/composer/autoload_static.php

apps/files/lib/AppInfo/Application.php

apps/files/lib/Listener/RenderReferenceEventListener.php

apps/files/src/reference-files.js

apps/files/src/views/FileReferencePickerElement.vue

apps/files/src/views/ReferenceFileWidget.vue

...te/Collaboration/Reference/File/FileReferenceProvider.php

lib/public/RichObjectStrings/Definitions.php

webpack.modules.js