Compute dashboard stats

Track open rate for new post notifications (user clicked the button in
the link or not). No data about the user is collected during the
process, it only uses an ID inserted by the issue delivery worker.

.sqlx/query-06f07a7522f3ee8e2cdfe5a7988a46f9a2598aa9c0618d00f6287978d5ce28ca.json

@@ -0,0 +1,20 @@
+{
+  "db_name": "PostgreSQL",
+  "query": "SELECT count(*) FROM notifications_delivered WHERE opened = TRUE",
+  "describe": {
+    "columns": [
+      {
+        "ordinal": 0,
+        "name": "count",
+        "type_info": "Int8"
+      }
+    ],
+    "parameters": {
+      "Left": []
+    },
+    "nullable": [
+      null
+    ]
+  },
+  "hash": "06f07a7522f3ee8e2cdfe5a7988a46f9a2598aa9c0618d00f6287978d5ce28ca"
+}