[IMP] web,*: config: add XXS size to config.device

In Less, the size_class XXS is defined and used in media queries.
This rev. adds it in JS for sake of consistency.

Code in addons comparing the size_class needed to be adapted due
to the new XXS size_class.

Moreover, we now use the helper 'config.device.isMobile' everywhere
we should, instead of manually comparing the size_class.

Finally, we changed the way the config.device has to be specified
in the test environment. From now on, only the size_class should be
set, and the isMobile flag is computed automatically.

addons/web/static/src/js/fields/basic_fields.js

@@ -1037,7 +1037,7 @@ var FieldPhone = FieldEmail.extend({
      * @private
      */
     _canCall: function () {
-        return config.device.size_class <= config.device.SIZES.XS;
+        return config.device.isMobile;
     }
 });