The `enqueue_jobs` is not correctly post-processing the post since the
post is being created inside a transaction block. This commit explicitly
enqueues the job outside transaction block.
When the linked topic is created we'll not hardcode the topic title and
let onebox work its magic instead so that the title can be updated
automatically.
This commit adds a site setting `auto_close_topics_create_linked_topic`
which when enabled works in conjunction with `auto_close_topics_post_count`
setting and creates a new linked topic for the topic just closed.
The auto-created new topic contains a link for all the previous topics
and the topic titles are appended with `(Part {n})`.
The setting is enabled by default.
