fix: downcase email before finding (#8921)

* fix: downcase email when finding

* feat: add `from_email` class

* refactor: use `from_email`

* feat: add rule to disallow find_by email directly

* chore:  remove redundant test

Since the previous imlpmentation didn't do a case-insentive search, a new user would be created, and the error would be raised at the DB layer. With the new changes, this test case is redundant

* refactor: use from_email

app/models/contact.rb

@@ -165,6 +165,12 @@ class Contact < ApplicationRecord
     email_format
   end
 
+  def self.from_email(email)
+    # rubocop:disable UseFromEmail,Migration/DepartmentName
+    find_by(email: email.downcase)
+    # rubocop:enable UseFromEmail,Migration/DepartmentName
+  end
+
   private
 
   def ip_lookup

app/models/user.rb

@@ -156,6 +156,12 @@ class User < ApplicationRecord
     mutations_from_database.changed?('email')
   end
 
+  def self.from_email(email)
+    # rubocop:disable UseFromEmail,Migration/DepartmentName
+    find_by(email: email.downcase)
+    # rubocop:enable UseFromEmail,Migration/DepartmentName
+  end
+
   private
 
   def remove_macros