Allow disabling "Browse server" button

To disable the "Browse server" button the `filebrowserBrowseUrl` must be set to `''` (empty string, see also http://ckeditor.com/forums/Support/Browse-server-button). Currently that setting is hardcoded in the widget. This small change allows disabling it by setting an empty string in the `CKEDITOR_CONFIGS` or even override the path to not use the built in views.

    CKEDITOR_CONFIGS = {
        'default': {
            # disable file browser and upload buttons
            'filebrowserBrowseUrl': '',
            'filebrowserUploadUrl': '',
        },
    }