Add canonical e-mail blocks for suspended accounts (#16049)

Prevent new accounts from being created using the same underlying
e-mail as a suspended account using extensions and period
permutations. Stores e-mails as a SHA256 hash

app/helpers/email_helper.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+module EmailHelper
+  def self.included(base)
+    base.extend(self)
+  end
+
+  def email_to_canonical_email(str)
+    username, domain = str.downcase.split('@', 2)
+    username, = username.gsub('.', '').split('+', 2)
+
+    "#{username}@#{domain}"
+  end
+
+  def email_to_canonical_email_hash(str)
+    Digest::SHA2.new(256).hexdigest(email_to_canonical_email(str))
+  end
+end