[FIX] portal, web_editor: keep BS 'img-responsive' *opt-in*

Bootstrap 'img-responsive' behavior was opt-in for a reason but commit
https://github.com/odoo/odoo/commit/f79b00c3b71e02f73537cb17285a6902fc0906b5
tried to defy that anyway. This would break existing client databases
... or at least our odoo.com.

This commit adds the img-responsive class for *new* *editor* images
(still with inline-block display), preventing breaking libraries or
custom (prod) images.

addons/portal/static/src/less/portal.less

@@ -152,9 +152,10 @@ img, .media_iframe_video, .o_image {
         margin-right: @grid-gutter-width / 2;
     }
 }
-img {
-    // Make all images responsive by default but with inline-block display
-    .img-responsive(inline-block);
+img.o_we_custom_image {
+    // Images added with the editor are .img-responsive by default but should
+    // still behave like inline content.
+    display: inline-block;
 }
 
 // Others

addons/web_editor/static/src/js/widgets/widgets.js

@@ -308,7 +308,10 @@ var ImageWidget = MediaWidget.extend({
                     img.src += _.str.sprintf('?access_token=%s', img.access_token);
                 }
                 if (!self.$media.is('img')) {
-                    self._replaceMedia($('<img/>'));
+                    // Note: by default the images receive the bootstrap opt-in
+                    // img-responsive class. We cannot make them all responsive
+                    // by design because of libraries and client databases img.
+                    self._replaceMedia($('<img/>', {class: 'img-responsive o_we_custom_image'}));
                 }
                 self.$media.attr('src', img.src);