add shuffle button to album and playlist pages

backend/util/util.go

@@ -0,0 +1,20 @@
+package util
+
+import (
+	"math/rand"
+	"time"
+)
+
+// Return a slice of range [0, n)
+func Range(n int) []int {
+	s := make([]int, n)
+	for i := 0; i < n; i++ {
+		s[i] = i
+	}
+	return s
+}
+
+func ShuffleSlice(a []int) {
+	rand.Seed(time.Now().UnixNano())
+	rand.Shuffle(len(a), func(i, j int) { a[i], a[j] = a[j], a[i] })
+}