fix: Paginate attachments API (#11044)

There are attachments with over 1000 attachments (unusual) in
production, and some of them timeout. This PR would limit the number of
attachments to 100 (which is sufficient for viewing the files in the
gallery, pagination on the UI can be added later).

app/views/api/v1/accounts/conversations/attachments.json.jbuilder

@@ -1,3 +1,7 @@
+json.meta do
+  json.total_count @attachments_count
+end
+
 json.payload @attachments do |attachment|
   json.message_id attachment.push_event_data[:message_id]
   json.thumb_url attachment.push_event_data[:thumb_url]