Remove NULL possibility from customer flag

It should only be true or false. This was flagged by Rubocop. I also
added another Rubocop suggestion and combined two migrations because
they are related.

db/migrate/20230516072511_add_created_manually_flag_to_customer.rb

@@ -1,5 +1,8 @@
+# frozen_string_literal: true
+
 class AddCreatedManuallyFlagToCustomer < ActiveRecord::Migration[7.0]
   def change
-    add_column :customers, :created_manually, :boolean, default: false
+    add_column :customers, :created_manually, :boolean, null: false, default: false
+    add_index :customers, :created_manually
   end
 end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[7.0].define(version: 2023_05_31_010957) do
+ActiveRecord::Schema[7.0].define(version: 2023_06_05_133804) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_stat_statements"
   enable_extension "plpgsql"
@@ -93,7 +93,7 @@
     t.datetime "terms_and_conditions_accepted_at", precision: nil
     t.string "first_name", default: "", null: false
     t.string "last_name", default: "", null: false
-    t.boolean "created_manually", default: false
+    t.boolean "created_manually", default: false, null: false
     t.index ["bill_address_id"], name: "index_customers_on_bill_address_id"
     t.index ["created_manually"], name: "index_customers_on_created_manually"
     t.index ["email"], name: "index_customers_on_email"