feat: notify users when chats are shared (#26914)

coderd/database/migrations/000538_chat_shared_notification.down.sql

@@ -0,0 +1 @@
+DELETE FROM notification_templates WHERE id = 'b789bd75-d7c6-4cab-9757-1147ab184903';

coderd/database/migrations/000538_chat_shared_notification.up.sql

@@ -0,0 +1,27 @@
+INSERT INTO notification_templates (
+    id,
+    name,
+    title_template,
+    body_template,
+    actions,
+    "group",
+    method,
+    kind,
+    enabled_by_default
+)
+VALUES (
+    'b789bd75-d7c6-4cab-9757-1147ab184903',
+    'Chat Shared',
+    E'{{.Labels.initiator}} shared a chat with you',
+    E'{{.Labels.initiator}} shared the chat "**{{.Labels.chat_title}}**" with you.',
+    '[
+        {
+            "label": "View chat",
+            "url": "{{base_url}}/agents/{{.Labels.chat_id}}"
+        }
+    ]'::jsonb,
+    'Chat Events',
+    NULL,
+    'system'::notification_template_kind,
+    true
+);