feat: use of imap login as default if present (#10249)

When moving form using Gmail Legacy auth to using OAuth, we need the
email address that will be used to connect. This is because we need to
store this email address in the cache and reuse when we get the callback
to find the associated inbox.

However there are cases where the imap login might be
`support@company.com` and the email used to communicate will be
`contact@company.com` (Probably an alias) In that case, we need to send
the correct email address to Chatwoot when re-authenticating

At the moment, we used the inbox email. This PR adds a check that
defaults to to `imap_login` if that is available and imap is enabled

This PR also fixes an unrelated problem where the email inbox creation
flow was not working

---

Tested it, it is working correctly

![CleanShot 2024-10-09 at 14 23
47@2x](https://github.com/user-attachments/assets/0e2cb6c8-1224-4b45-b34a-7b19611249bc)

app/controllers/oauth_callback_controller.rb

@@ -25,7 +25,7 @@ class OauthCallbackController < ApplicationController
   end
 
   def find_or_create_inbox
-    channel_email = Channel::Email.find_by(email: users_data['email'], account: account)
+    channel_email = find_channel_by_email
     # we need this value to know where to redirect on sucessful processing of the callback
     channel_exists = channel_email.present?
 
@@ -39,6 +39,10 @@ class OauthCallbackController < ApplicationController
     [channel_email.inbox, channel_exists]
   end
 
+  def find_channel_by_email
+    Channel::Email.find_by(email: users_data['email'], account: account)
+  end
+
   def update_channel(channel_email)
     channel_email.update!({
                             imap_login: users_data['email'], imap_address: imap_address,