Fixed #07470: Ability to specify a custom css and a custom js per survey

I went kind of lazy on this one. Instead of making some more configurations, I chose to simply pick up the files already available as resources for the survey.

When files with .css extension are encountered in the survey resources (upload/surveys/619513/files), they are registered as CSS files for the application.

When files with .js extension are encountered in the survey resources (upload/surveys/619513/files), they are registered as script files for the application, to be appended at the end.