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

app/models/email_domain_block.rb

@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+# == Schema Information
+#
+# Table name: email_domain_blocks
+#
+#  id         :integer          not null, primary key
+#  domain     :string           not null
+#  created_at :datetime         not null
+#  updated_at :datetime         not null
+#
+
+class EmailDomainBlock < ApplicationRecord
+  def self.block?(email)
+    domain = email.gsub(/.+@([^.]+)/, '\1')
+    where(domain: domain).exists?
+  end
+end