Use relative path for media assets

[toracle]

I had a problem with asset URL, and inspected into it.

css and js property of django Media class is rendered by render_js and render_css method.

When look into that method, it calls absolute_path method to make a path absolute, and absolute_path calls static function, and it calls StaticNode.handle_simple method.

And, this handle_simple method do the same things you do, turn a path to absolute one with storage.url().

So, default django Media logic act like this:

1. If js or css path is an absolute path, use itself.
2. If they are relative one,
   1. If django.contrib.staticfiles app is installed, get a staticfile_storage which is defined by settings.STATICFILES_STORAGE value, and use storage.url(path) to convert it to absolute.
   2. If the app is not installed, just add STATIC_URL prefix to the path.

So, I think we can simply to remove any URL prefix and using a relative path would works well.

[frost-nzcr4]

Previous versions of Django (1.9-1.11) use the same logic, so STATIC_URL should be ommited.

[submarcos]

I confirm STATIC_URL should be removed. It doesn't work with external file storage backend (example: s3). I need to patch with

from django_json_widget.widgets import JSONEditorWidget

class JSONEditorWidget(JSONEditorWidget):
    """
    Override media infos ith compliant django style
    Wait for https://github.com/jmrivas86/django-json-widget/pull/23 merge
    """
    class Media(JSONEditorWidget.Media):
        css = {'all': ('dist/jsoneditor.min.css',)}
        js = ('dist/jsoneditor.min.js',)

[jmrivas86]

I close this PR because is solved in #30