Add User#email_domain method to extract domain from email address (#35159)

spec/models/user_spec.rb

@@ -166,6 +166,34 @@ RSpec.describe User do
     end
   end
 
+  describe '#email_domain' do
+    subject { described_class.new(email: email).email_domain }
+
+    context 'when value is nil' do
+      let(:email) { nil }
+
+      it { is_expected.to be_nil }
+    end
+
+    context 'when value is blank' do
+      let(:email) { '' }
+
+      it { is_expected.to be_nil }
+    end
+
+    context 'when value has valid domain' do
+      let(:email) { 'user@host.example' }
+
+      it { is_expected.to eq('host.example') }
+    end
+
+    context 'when value has no split' do
+      let(:email) { 'user$host.example' }
+
+      it { is_expected.to be_nil }
+    end
+  end
+
   describe '#update_sign_in!' do
     context 'with an existing user' do
       let!(:user) { Fabricate :user, last_sign_in_at: 10.days.ago, current_sign_in_at: 1.hour.ago, sign_in_count: 123 }