Add new fields to signature

- Add :date_of_birth and :postal_code to Signature to allow send these
  fields to CustomCensusAPI

- Add new model presence validates: Only validate :date_of_birth and
  :postal_code presence when the application has configured Remote Census
  and their alias fields has values.

db/migrate/20190429214633_add_date_of_birth_to_signatures.rb

@@ -0,0 +1,5 @@
+class AddDateOfBirthToSignatures < ActiveRecord::Migration
+  def change
+    add_column :signatures, :date_of_birth, :date
+  end
+end

db/migrate/20190430003908_add_postal_code_to_signatures.rb

@@ -0,0 +1,5 @@
+class AddPostalCodeToSignatures < ActiveRecord::Migration
+  def change
+    add_column :signatures, :postal_code, :string
+  end
+end

db/schema.rb

@@ -1476,6 +1476,8 @@ ActiveRecord::Schema.define(version: 20190607160900) do
     t.boolean  "verified",           default: false
     t.datetime "created_at"
     t.datetime "updated_at"
+    t.date     "date_of_birth"
+    t.string   "postal_code"
   end
 
   create_table "site_customization_content_blocks", force: :cascade do |t|