Closes #9416: Dashboard widgets (#11823)

* Replace masonry with gridstack

* Initial work on dashboard widgets

* Implement function to save dashboard layout

* Define a default dashboard

* Clean up widgets

* Implement widget configuration views & forms

* Permit merging dict value with existing dict in user config

* Add widget deletion view

* Enable HTMX for widget configuration

* Implement view to add dashboard widgets

* ObjectCountsWidget: Identify models by app_label & name

* Add color customization to dashboard widgets

* Introduce Dashboard model to store user dashboard layout & config

* Clean up utility functions

* Remove hard-coded API URL

* Use fixed grid cell height

* Add modal close button

* Clean up dashboard views

* Rebuild JS

netbox/extras/api/serializers.py

@@ -34,6 +34,7 @@ __all__ = (
     'ContentTypeSerializer',
     'CustomFieldSerializer',
     'CustomLinkSerializer',
+    'DashboardSerializer',
     'ExportTemplateSerializer',
     'ImageAttachmentSerializer',
     'JobResultSerializer',
@@ -563,3 +564,13 @@ class ContentTypeSerializer(BaseModelSerializer):
     class Meta:
         model = ContentType
         fields = ['id', 'url', 'display', 'app_label', 'model']
+
+
+#
+# User dashboard
+#
+
+class DashboardSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = Dashboard
+        fields = ('layout', 'config')