Enable more eslint-plugin-react rules

This should contain all the rules we probably want to use, except
react/wrap-multilines, which requires a larger codemod, and would
clutter this commit.

_js/examples/.eslintrc

@@ -1,3 +1,6 @@
+---
 rules:
-  block-scoped-var: false
-  no-undef: false
+  block-scoped-var: 0
+  no-undef: 0
+  react/react-in-jsx-scope: 0
+  react/jsx-no-undef: 0