Redirect en.reactjs.org to reactjs.org (#3446)

Currently this resolves to the same Netlify site, but is an alias and just serves the same content. There's no good reason to support both domains, so redirect.

This pattern should not be copied into any translation sites.

static/_redirects

@@ -1,3 +1,7 @@
+# Ensure English site redirects to Apex. While not used directly by us, may be linked elsewhere.
+http://en.reactjs.org/* https://reactjs.org/:splat 301!
+https://en.reactjs.org/* https://reactjs.org/:splat 301!
+
 /tips/controlled-input-null-value.html  /docs/forms.html#controlled-input-null-value
 /concurrent     /docs/concurrent-mode-intro.html
 /hooks          /docs/hooks-intro.html
@@ -35,4 +39,4 @@
 /link/test-utils-mock-component     https://gist.github.com/bvaughn/fbf41b3f895bf2d297935faa5525eee9  301
 /link/uselayouteffect-ssr           https://gist.github.com/gaearon/e7d97cdf38a2907924ea12e4ebdf3c85  301
 /link/react-devtools-faq            https://github.com/facebook/react/tree/master/packages/react-devtools#faq 301
-/link/setstate-in-render            https://github.com/facebook/react/issues/18178#issuecomment-595846312 301
+/link/setstate-in-render            https://github.com/facebook/react/issues/18178#issuecomment-595846312 301