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

lib/integrations/slack/slack_message_helper.rb

@@ -69,7 +69,7 @@ module Integrations::Slack::SlackMessageHelper
 
   def sender
     user_email = slack_client.users_info(user: params[:event][:user])[:user][:profile][:email]
-    conversation.account.users.find_by(email: user_email)
+    conversation.account.users.from_email(user_email)
   end
 
   def private_note?