fix(React IS): fix back button behaviour with basic and seo friendly routing (#337)

* fix(React IS): fix back button behaviour with seo friendly routing

This change fixes the behaviour of the back button in SEO friendly routing example
Prior to this change, the example was missing the bit that updates the
externally managed searchState when location changes:

```jsx
  const [searchState, setSearchState] = useState(urlToSearchState(location));

  useEffect(() => {
    setSearchState(urlToSearchState(location));
  }, [location]);
```

fixes [FX-246](https://algolia.atlassian.net/browse/FX-246)

* fix(React IS): fix back button behaviour with basic routing

* useRef instead of variable

Author: tkrugg

React InstantSearch/routing-basic/src/App.js

@@ -1,4 +1,4 @@
-import React, { useRef, useState } from 'react';
+import React, { useEffect, useRef, useState } from 'react';
 import {
   InstantSearch,
   Hits,
@@ -39,6 +39,10 @@ export function App({ location, history }) {
     setSearchState(updatedSearchState);
   }
 
+  useEffect(() => {
+    setSearchState(urlToSearchState(location));
+  }, [location]);
+
   return (
     <div className="container">
       <InstantSearch

React InstantSearch/routing-seo-friendly/src/App.js

@@ -1,4 +1,4 @@
-import React, { useState } from 'react';
+import React, { useEffect, useRef, useState } from 'react';
 import {
   InstantSearch,
   Hits,
@@ -135,20 +135,22 @@ Hit.propTypes = {
 
 const App = ({ location, history }) => {
   const [searchState, setSearchState] = useState(urlToSearchState(location));
-  const [debouncedSetState, setDebouncedSetState] = useState(null);
+  const debouncedSetStateRef = useRef(null);
 
   const onSearchStateChange = updatedSearchState => {
-    clearTimeout(debouncedSetState);
+    clearTimeout(debouncedSetStateRef.current);
 
-    setDebouncedSetState(
-      setTimeout(() => {
-        history.push(searchStateToUrl(updatedSearchState), updatedSearchState);
-      }, DEBOUNCE_TIME)
-    );
+    debouncedSetStateRef.current = setTimeout(() => {
+      history.push(searchStateToUrl(updatedSearchState), updatedSearchState);
+    }, DEBOUNCE_TIME);
 
     setSearchState(updatedSearchState);
   };
 
+  useEffect(() => {
+    setSearchState(urlToSearchState(location));
+  }, [location]);
+
   return (
     <div className="container">
       <InstantSearch