users: add is_deleted flag.

Add is_deleted flag to users table in preparation for fixing #4555.

db/migrate/20221106062419_add_is_deleted_to_users.rb

@@ -0,0 +1,8 @@
+class AddIsDeletedToUsers < ActiveRecord::Migration[7.0]
+  disable_ddl_transaction!
+
+  def change
+    add_column :users, :is_deleted, :boolean, null: false, default: false
+    add_index :users, :is_deleted, where: "is_deleted = TRUE", algorithm: :concurrently
+  end
+end