Add cold-start follow recommendations (#15945)

db/migrate/20210322164601_create_account_summaries.rb

@@ -0,0 +1,9 @@
+class CreateAccountSummaries < ActiveRecord::Migration[5.2]
+  def change
+    create_view :account_summaries, materialized: true
+
+    # To be able to refresh the view concurrently,
+    # at least one unique index is required
+    safety_assured { add_index :account_summaries, :account_id, unique: true }
+  end
+end