feat(perf): improve performance of conversation filter API [CW-1605] (#7120)

* feat(perf): don't use count

* feat(perf): include messages in the conversation query

* feat: include contact inboxes

* refactor: use blank?

app/views/api/v1/conversations/partials/_conversation.json.jbuilder

@@ -17,7 +17,7 @@ json.meta do
 end
 
 json.id conversation.display_id
-if conversation.messages.count.zero?
+if conversation.messages.first.blank?
   json.messages []
 elsif conversation.unread_incoming_messages.count.zero?
   json.messages [conversation.messages.includes([{ attachments: [{ file_attachment: [:blob] }] }]).last.try(:push_event_data)]