Few people used forum subscriptions (only around 100), and even fewer
people were subscribed to active threads. Most subscriptions were for
old threads that will never be bumped again. The implementation also had
a few problems:
* Unsubscribe links in emails didn't work (they unset the user's
receive_email_notifications flag, but forum subscriptions didn't
respect this flag).
* Some users had invalid email addresses, which caused notifications to
bounce. There was no mechanism for preventing bounces.
* The implementation wasn't scalable. It involved a daily linear scan
over _all_ forum subscriptions looking for any topics that had been updated.
* Move the Curated pool updater from Reportbooru to Danbooru.
* Change the process for selecting curated posts. Previously it was
every post from the last week with at least three supervotes. This was
flawed because it included both super-upvotes and super-downvotes. Now
it's the top 100 posts from the last week, ordered from most super-upvoted
to least.
Disable database timeouts durings daily maintenance. Fixes
`regenerate_post_counts!` timing out. Remove calls to without_timeout
because otherwise it will reenable the timeout when trying to restore
the old timeout (see 97cc873a3f).
* Automatically fix all tags with incorrect counts during daily
maintenance (previously only tags with negative counts were fixed).
* Log fixed tags to NewRelic.
* Remove the ability to manually fix tag counts with the "Fix" button on
the /tags listing. This is no longer necessary now that tags are
fixed automatically.
Drop support for https://danbooru.donmai.us/cache/tags.json. This was a
nightly dump of the tags table that was originally added in #1012. It
was never documented and never really used except for by the DanbooruUp
extension.
Setting the statement timeout at the beginning didn't work because
`PostPruner.new.prune!` clobbers the timeout (it calls `without_timeout`,
which doesn't restore the timeout properly if the timeout was zero).
