Enable a couple of return related ESLint rules

Please find additional details at:
 - https://eslint.org/docs/rules/no-return-await
 - https://eslint.org/docs/rules/no-useless-return

.eslintrc

@@ -87,6 +87,7 @@
     "no-octal-escape": "error",
     "no-octal": "error",
     "no-redeclare": "error",
+    "no-return-await": "error",
     "no-self-assign": "error",
     "no-self-compare": "error",
     "no-throw-literal": "error",
@@ -95,6 +96,7 @@
     "no-useless-call": "error",
     "no-useless-catch": "error",
     "no-useless-concat": "error",
+    "no-useless-return": "error",
     "prefer-promise-reject-errors": "error",
     "wrap-iife": ["error", "any"],
     "yoda": ["error", "never", {

web/pdf_print_service.js

@@ -292,7 +292,6 @@ window.addEventListener('keydown', function(event) {
     } else {
       event.stopPropagation();
     }
-    return;
   }
 }, true);
 if (hasAttachEvent) {