Add graphs and retention metrics to admin dashboard (#16829)

app/serializers/rest/admin/cohort_serializer.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+class REST::Admin::CohortSerializer < ActiveModel::Serializer
+  attributes :period, :frequency
+
+  class CohortDataSerializer < ActiveModel::Serializer
+    attributes :date, :percent, :value
+
+    def date
+      object.date.iso8601
+    end
+  end
+
+  has_many :data, serializer: CohortDataSerializer
+
+  def period
+    object.period.iso8601
+  end
+end