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/services/data_import/contact_manager.rb

@@ -35,7 +35,7 @@ class DataImport::ContactManager
   def find_contact_by_email(params)
     return unless params[:email]
 
-    @account.contacts.find_by(email: params[:email])
+    @account.contacts.from_email(params[:email])
   end
 
   def find_contact_by_phone_number(params)