Add AI tag model and UI.

Add a database model for storing AI-predicted tags, and add a UI for browsing and searching these tags.

AI tags are generated by the Danbooru Autotagger (https://github.com/danbooru/autotagger). See that
repo for details about the model.

The database schema is `ai_tags (media_asset_id integer, tag_id integer, score smallint)`. This is
designed to be as space-efficient as possible, since in production we have over 300 million
AI-generated tags (6 million images and 50 tags per post). This amounts to over 10GB in size, plus
indexes.

You can search for AI tags using e.g. `ai:scenery`. You can do `ai:scenery -scenery` to find posts
where the scenery tag is potentially missing, or `scenery -ai:scenery` to find posts that are
potentially mistagged (or more likely where the AI missed the tag).

You can browse AI tags at https://danbooru.donmai.us/ai_tags. On this page you can filter by
confidence level. You can also search unposted media assets by AI tag.

To generate tags, use the `autotag` script from the Autotagger repo, something like this:

  docker run --rm -v ~/danbooru/public/data/360x360:/images ghcr.io/danbooru/autotagger ./autotag -c -f /images | gzip > tags.csv.gz

To import tags, use the fix script in script/fixes/. Expect a Danbooru-size dataset to take
hours to days to generate tags, then 20-30 minutes to import. Currently this all has to be done by hand.

app/logical/concerns/searchable.rb

@@ -430,7 +430,13 @@ module Searchable
     end
 
     if model == Post && params["#{attr}_tags_match"].present?
-      relation = relation.where(attr => Post.user_tag_match(params["#{attr}_tags_match"], current_user).reorder(nil))
+      posts = Post.user_tag_match(params["#{attr}_tags_match"], current_user).reorder(nil)
+
+      if association.through_reflection?
+        relation = relation.includes(association.through_reflection.name).where(association.through_reflection.name => { attr => posts })
+      else
+        relation = relation.where(attr => posts)
+      end
     end
 
     if params["has_#{attr}"].to_s.truthy? || params["has_#{attr}"].to_s.falsy?