TNL-3258 Reset group configuration model when a child content group is deleted (+5 squashed commits)

Squashed commits:
[450761c] found an alternative way to restore model when any of the nested model is deleted
[1b7eb85] passed the context to the inner functions
[c87dafd] refactored the model access using encapsulation instead of directly accessing it
[5d025ff] return the promise
[d8d1f50] fixup! quality (+1 squashed commit)
Squashed commits:
[daa7f81] reset the configuration when the model is deleted (+1 squashed commit)
Squashed commits:
[c7f41f7] reverted: skip test_can_delete_unused_content_group due to flakiness

cms/static/js/models/group_configuration.js

@@ -36,8 +36,11 @@ function(Backbone, _, str, gettext, GroupModel, GroupCollection) {
         }],
 
         initialize: function(attributes, options) {
+            this.on('remove:groups', this.groupRemoved);
+
             this.canBeEmpty = options && options.canBeEmpty;
             this.setOriginalAttributes();
+
             return this;
         },
 
@@ -120,6 +123,10 @@ function(Backbone, _, str, gettext, GroupModel, GroupCollection) {
                     };
                 }
             }
+        },
+
+        groupRemoved: function () {
+            this.setOriginalAttributes();
         }
     });