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/validators/blacklisted_email_validator.rb

@@ -12,6 +12,7 @@ class BlacklistedEmailValidator < ActiveModel::Validator
   end
 
   def on_blacklist?(value)
+    return true if EmailDomainBlock.block?(value)
     return false if Rails.configuration.x.email_domains_blacklist.blank?
 
     domains = Rails.configuration.x.email_domains_blacklist.gsub('.', '\.')