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/migrations/0087_dashboard.py

@@ -0,0 +1,25 @@
+# Generated by Django 4.1.7 on 2023-02-24 00:56
+
+from django.conf import settings
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        migrations.swappable_dependency(settings.AUTH_USER_MODEL),
+        ('extras', '0086_configtemplate'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='Dashboard',
+            fields=[
+                ('id', models.BigAutoField(auto_created=True, primary_key=True, serialize=False)),
+                ('layout', models.JSONField()),
+                ('config', models.JSONField()),
+                ('user', models.OneToOneField(on_delete=django.db.models.deletion.CASCADE, related_name='dashboard', to=settings.AUTH_USER_MODEL)),
+            ],
+        ),
+    ]