Add last_active_at to Members

This is an attribute we will be updating when
a member either registers to vote, or self-confirms
their membership active status.

db/migrate/20220116212309_add_last_active_at_to_members.rb

@@ -0,0 +1,5 @@
+class AddLastActiveAtToMembers < ActiveRecord::Migration[6.1]
+  def change
+    add_column :members, :last_active_at, :timestamp, null: true
+  end
+end