Implement Firestore database on exchange-rates collection

_includes/body/header/exchange-rates/usd.html

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+<span class="glyphicon glyphicon-globe"></span>
+<span id="sell-usd-exchange-rates"></span>
+<span class="glyphicon glyphicon-info-sign"></span>
+<span id="updated-at-usd-exchange-rates"></span>
+
+<script>
+  function capitalize(string) {
+    return string.charAt(0).toUpperCase() + string.slice(1);
+  }
+
+  function monthName(date, locale, type){
+    name = date.toLocaleString(locale, { month: type });
+    return capitalize(name);
+  }
+
+  function parseDate(date){
+    return  date.getDate() + " / " +
+            monthName(date, 'es-MX', "short") + " / " +
+            date.getFullYear() + " " +
+            ((date.getHours()%12)?(date.getHours()%12):12) + ":" +
+            (date.getMinutes()<10?'0':'') + date.getMinutes() +
+            (date.getHours()>=12?' PM':' AM');
+  }
+
+  getRealtimeExchangeRates = function() {
+    const docRef = firestoredb.doc("exchange-rates/usd");
+    const usdSell = document.querySelector("#sell-usd-exchange-rates");
+    const usdUpdatedAt = document.querySelector("#updated-at-usd-exchange-rates");
+
+    docRef.onSnapshot(function (doc){
+      if (doc && doc.exists) {
+        const data = doc.data();
+        usdSell.innerText = "USD: " + data.sell.toFixed(2);
+        usdUpdatedAt.innerText = parseDate(new Date(data.updated_at));
+      }
+    });
+  }
+
+  getRealtimeExchangeRates();
+</script>

_includes/head/firebase/sdk.html

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+<script src="https://www.gstatic.com/firebasejs/4.11.0/firebase.js"></script>
+<script src="https://www.gstatic.com/firebasejs/4.11.0/firebase-firestore.js"></script>
+<script src="/js/firebase/firebase-sdk.js"></script>