web_widget_image_url: add img class same as std

Add html class to the widget in order to behave the same as FieldBinaryWidget-img.
This allows to respect the colspan rules and contains large images
without breaking the view.

web_widget_image_url/static/src/xml/web_widget_image_url.xml

@@ -6,7 +6,7 @@
 <templates xml:space="preserve">
     <t t-name="FieldImageURL">
         <span class="oe_form_field oe_form_field_image" t-att-style="widget.attrs.style">
-            <img t-att-src="widget.url()"
+            <img class="img img-fluid" t-att-src="widget.url()"
             t-att-style="(!widget.readonly ? 'border: 0' : 'border: 1')"
             t-att-name="widget.name"
             t-att-width="widget.attrs.img_width || widget.attrs.width"