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

spec/models/email_domain_block_spec.rb

@@ -0,0 +1,21 @@
+require 'rails_helper'
+
+RSpec.describe EmailDomainBlock, type: :model do
+  describe 'validations' do
+    it 'has a valid fabricator' do
+      email_domain_block = Fabricate.build(:email_domain_block)
+      expect(email_domain_block).to be_valid
+    end
+  end
+
+  describe 'block?' do
+    it 'returns true if the domain is registed' do
+      Fabricate(:email_domain_block, domain: 'example.com')
+      expect(EmailDomainBlock.block?('nyarn@example.com')).to eq true
+    end
+    it 'returns true if the domain is not registed' do
+      Fabricate(:email_domain_block, domain: 'domain')
+      expect(EmailDomainBlock.block?('example')).to eq false
+    end
+  end
+end