Pull out livestream data-fetch into an effect.

Differences with the original from 'livestreamList/view.jsx':

- Returns null instead of an empty object for the empty case. This removes the need for clients to create a temp array to determine if the list is empty ('Object.keys(x).length').

- Allow option to not periodically refresh by setting the interval to 0.

ui/effects/use-get-livestreams.js

@@ -0,0 +1,52 @@
+// @flow
+import React from 'react';
+import { BITWAVE_LIVE_API } from 'constants/livestream';
+
+/**
+ * Gets latest livestream info list. Returns null (instead of a blank object)
+ * when there are no active livestreams.
+ *
+ * @param refreshMs
+ * @returns {{livestreamMap: null, loading: boolean}}
+ */
+export default function useGetLivestreams(refreshMs: number) {
+  const [loading, setLoading] = React.useState(true);
+  const [livestreamMap, setLivestreamMap] = React.useState(null);
+
+  React.useEffect(() => {
+    function checkCurrentLivestreams() {
+      fetch(BITWAVE_LIVE_API)
+        .then((res) => res.json())
+        .then((res) => {
+          setLoading(false);
+          if (!res.data) {
+            setLivestreamMap(null);
+            return;
+          }
+
+          const livestreamMap = res.data.reduce((acc, curr) => {
+            acc[curr.claimId] = curr;
+            return acc;
+          }, {});
+
+          setLivestreamMap(livestreamMap);
+        })
+        .catch((err) => {
+          setLoading(false);
+        });
+    }
+
+    checkCurrentLivestreams();
+
+    if (refreshMs > 0) {
+      let fetchInterval = setInterval(checkCurrentLivestreams, refreshMs);
+      return () => {
+        if (fetchInterval) {
+          clearInterval(fetchInterval);
+        }
+      };
+    }
+  }, []);
+
+  return { livestreamMap, loading };
+}