eslint: prevent async forEach(callbackFn) (#1669)

Closes getodk/central#1456

Author: alxndrsn

.eslintrc.json

@@ -29,7 +29,12 @@
     "no-multiple-empty-lines": "off",
     "no-nested-ternary": "off",
     "no-only-tests/no-only-tests": [ "error", { "block": [ "describe", "it", "describeMigration" ] } ],
-    "no-restricted-syntax": "off",
+    "no-restricted-syntax": [ "error",
+      {
+        "message": "Do not use async functions in .forEach()",
+        "selector": "CallExpression[callee.property.name='forEach'] > :matches(ArrowFunctionExpression, FunctionExpression)[async=true].arguments"
+      }
+    ],
     "no-underscore-dangle": "off",
     "nonblock-statement-body-position": "off",
     "object-curly-newline": [ "error", {