Support sending hidden read receipts

Fixes https://github.com/vector-im/riot-web/issues/2527

src/settings/Settings.js

@@ -394,4 +394,11 @@ export const SETTINGS = {
         // This is a tri-state value, where `null` means "prompt the user".
         default: null,
     },
+    "sendReadReceipts": {
+        supportedLevels: LEVELS_ACCOUNT_SETTINGS,
+        displayName: _td(
+            "Send read receipts for messages (requires compatible homeserver to disable)",
+        ),
+        default: true,
+    },
 };