Implement EmailBlackList (#5109)

* Implement BlacklistedEmailDomain

* Use Faker::Internet.domain_name

* Remove note column

* Add frozen_string_literal comment

* Delete unnecessary codes

* Sort alphabetically

* Change of wording

* Rename BlacklistedEmailDomain to EmailDomainBlock

db/migrate/20170928082043_create_email_domain_blocks.rb

@@ -0,0 +1,9 @@
+class CreateEmailDomainBlocks < ActiveRecord::Migration[5.1]
+  def change
+    create_table :email_domain_blocks do |t|
+      t.string :domain, null: false
+
+      t.timestamps
+    end
+  end
+end