settings: mark UserSettings as changed in KeychainStorage properties

Maybe this isn't strictly needed at the moment, but to remain consistent
with the Settings and StringSettings property wrapper we will do this.

damus/Util/KeychainStorage.swift

@@ -64,6 +64,8 @@ import Security
                 
                 _ = SecItemDelete(query)
             }
+                
+            UserSettingsStore.shared!.objectWillChange.send()
         }
     }