PERF: Add index for (action_type,created_at)` on `user_actions` table.

Speeds up counting of likes in `About#stats`.

db/migrate/20190405044140_add_index_action_type_created_at_on_user_actions.rb

@@ -0,0 +1,5 @@
+class AddIndexActionTypeCreatedAtOnUserActions < ActiveRecord::Migration[5.2]
+  def change
+    add_index :user_actions, [:action_type, :created_at]
+  end
+end