Fix old type MPIMs not being marked as unread on start
Fix old type MPIMs not being marked as unread on start

Old type MPIMs (has `is_group: true`) has `is_open`, but not
`unread_count_display` in the response from `conversations.list`, so
they were not marked as unread on start if background_load_all_history
was set to false because we have to request the history, but
`check_should_open` created the buffer, so it wasn't requested here.

wee-slack/wee-slack