Add Storage security rules

storage.rules

@@ -0,0 +1,13 @@
+rules_version = '2';
+
+service firebase.storage {
+  match /b/{bucket}/o {
+    match /{userId}/{vocabId} {
+      allow read: if request.auth != null;
+      allow create, update: if request.auth.uid == userId &&
+        request.resource.contentType.matches("audio/mp3") &&
+        request.resource.size < 1024 * 1024 / 5 &&
+        vocabId.matches('[a-zA-Z0-9_-]+\\.mp3');
+    }
+  }
+}