Certain parts of comment rendering triggered sql queries that we didn't
really need to do. Rework things to avoid this.
* Preload comment creators in order to display commenter names with link_to_user.
* Preload comment votes in order to display "undo vote" links. Only preload
votes for members since anonymous users can't vote and don't have "undo
vote" links.
* Rework various conditionals to do the filtering in Ruby so that we
avoid issuing any extra queries in sql.
* Avoid issuing any queries at all when the post doesn't have any
comments (when last_commented_at is blank).
Changes:
* Drop Users.id_to_name.
* Don't cache Users.name_to_id.
* Replace calls to name_to_id with find_by_name when possible.
* Don't autodefine creator_name in belongs_to_creator.
* Don't autodefine updater_name in belongs_to_updater.
* Instead manually define creator_name / updater_name only on models that need
to return these fields in the api.
id_to_name was cached to reduce the impact of N+1 query patterns in
certain places, especially in api responses that return creator_name /
updater_name fields. But it still meant we were doing N calls to
memcache. Using `includes` to prefetch users avoids this N+1 pattern.
name_to_id had no need be cached, it was never used in any performance-
sensitive contexts.
Avoiding caching also avoids the need to keep these caches consistent.
This causes the browser to prefetch the next page in a search, which may
make browsing seem faster to the end user, but it effectively doubles
the load on the server.
Avoid some queries used in wiki page excerpts:
* Only try to load the artist if the tag is an artist tag.
* Avoid using `exists?` queries for wiki pages.
* Bugfix: don't show wiki excerpts for deleted wikis.
https://danbooru.donmai.us/forum_topics/9127?page=265#forum_post_153180:
> I'm not sure how to reproduce this, but sometimes on the upload page
> when I click the button to pop out the tag box, the screen scrolls up to
> the picture but the box doesn't pop out, and I have to scroll back down
> and click it again
https://danbooru.donmai.us/forum_topics/9127?page=266#forum_post_153183:
> Yes, this was brought up by Unbreakable on Discord a while back. It's
> due to the fact that the event handler that pops out the dialog box has
> not yet been bound by the time you click it. Since that clickable
> control has an empty anchor link of "#" which goes nowhere, it therefore
> redirects the user to the top of the page.
Fix exception in Pool#previous_post_id when the post id is not contained
in the pool.
This can happen when a post's pool_string lists the post as being in a
certain pool, but the post is not included in the pool's post_ids. Such
pool_string / post_ids inconsistencies exist because of past bugs.
