FEATURE: enforce_canonical_emails site setting

The new `enforce_canonical_emails` site setting ensures that emails in the
canonical form are unique.

This mean that if `s.a.m+1@gmail.com` is registered `sam@gmail.com` will
not be allowed.

The commit contains a blanket "tag strip" (stripping everything after +)
it also contains special handling of a "dot strip" for googlemail and gmail.

The setting only impacts new registrations after `enforce_canonical_emails`

The setting is default false so it will not impact any existing installs.

app/models/user_email.rb

@@ -15,10 +15,27 @@ class UserEmail < ActiveRecord::Base
   validate :user_id_not_changed, if: :primary
   validate :unique_email
 
+  before_save :save_canonical
+
   scope :secondary, -> { where(primary: false) }
 
+  def self.canonical(email)
+    name, domain = email.split('@', 2)
+    name = name.gsub(/\+.*/, '')
+    if ['gmail.com', 'googlemail.com'].include?(domain.downcase)
+      name = name.gsub('.', '')
+    end
+    "#{name}@#{domain}".downcase
+  end
+
   private
 
+  def save_canonical
+    if SiteSetting.enforce_canonical_emails && self.will_save_change_to_email?
+      self.canonical_email = UserEmail.canonical(self.email)
+    end
+  end
+
   def strip_downcase_email
     if self.email
       self.email = self.email.strip
@@ -32,8 +49,14 @@ class UserEmail < ActiveRecord::Base
   end
 
   def unique_email
-    if self.will_save_change_to_email? && self.class.where("lower(email) = ?", email).exists?
-      self.errors.add(:email, :taken)
+    if self.will_save_change_to_email?
+      exists = self.class.where("lower(email) = ?", email).exists?
+      exists ||= SiteSetting.enforce_canonical_emails &&
+        self.class.where("canonical_email = ?", UserEmail.canonical(email)).exists?
+
+      if exists
+        self.errors.add(:email, :taken)
+      end
     end
   end
 
@@ -50,15 +73,17 @@ end
 #
 # Table name: user_emails
 #
-#  id         :integer          not null, primary key
-#  user_id    :integer          not null
-#  email      :string(513)      not null
-#  primary    :boolean          default(FALSE), not null
-#  created_at :datetime         not null
-#  updated_at :datetime         not null
+#  id              :integer          not null, primary key
+#  user_id         :integer          not null
+#  email           :string(513)      not null
+#  primary         :boolean          default(FALSE), not null
+#  created_at      :datetime         not null
+#  updated_at      :datetime         not null
+#  canonical_email :string
 #
 # Indexes
 #
+#  index_user_emails_on_canonical_email      (canonical_email) WHERE (canonical_email IS NOT NULL)
 #  index_user_emails_on_email                (lower((email)::text)) UNIQUE
 #  index_user_emails_on_user_id              (user_id)
 #  index_user_emails_on_user_id_and_primary  (user_id,primary) UNIQUE WHERE "primary"