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.

config/initializers/inflections.rb

@@ -9,6 +9,7 @@ ActiveSupport::Inflector.inflections(:en) do |inflect|
   inflect.acronym "URL"
   inflect.acronym "URLs"
   inflect.acronym "AST"
+  inflect.acronym "AI"
 #   inflect.plural /^(ox)$/i, '\1en'
 #   inflect.singular /^(ox)en/i, '\1'
 #   inflect.irregular 'person', 'people'

config/routes.rb

@@ -158,6 +158,7 @@ Rails.application.routes.draw do
   end
   resources :media_assets, only: [:index, :show]
   resources :media_metadata, only: [:index]
+  resources :ai_tags, only: [:index]
   resources :mod_actions
   resources :moderation_reports, only: [:new, :create, :index, :show, :update]
   resources :modqueue, only: [:index]