FEATURE: Add email normalization rules setting (#14593)

When this setting is turned on, it will check that normalized emails
are unique. Normalized emails are emails without any dots or plus
aliases.

This setting can be used to block use of aliases of the same email
address.

spec/models/user_email_spec.rb

@@ -26,6 +26,32 @@ describe UserEmail do
     end
   end
 
+  describe 'normalized_email' do
+    it 'checks if normalized email is unique' do
+      SiteSetting.normalize_emails = true
+
+      user_email = user.user_emails.create(email: "a.b+c@example.com", primary: false)
+      expect(user_email.normalized_email).to eq("ab@example.com")
+      expect(user_email).to be_valid
+
+      user_email = user.user_emails.create(email: "a.b+d@example.com", primary: false)
+      expect(user_email.normalized_email).to eq("ab@example.com")
+      expect(user_email).not_to be_valid
+    end
+
+    it 'does not check uniqueness if email normalization is not enabled' do
+      SiteSetting.normalize_emails = false
+
+      user_email = user.user_emails.create(email: "a.b+c@example.com", primary: false)
+      expect(user_email.normalized_email).to eq("ab@example.com")
+      expect(user_email).to be_valid
+
+      user_email = user.user_emails.create(email: "a.b+d@example.com", primary: false)
+      expect(user_email.normalized_email).to eq("ab@example.com")
+      expect(user_email).to be_valid
+    end
+  end
+
   context "indexes" do
     it "allows only one primary email" do
       expect {