tags: track tag histories.

Track the history of the tag `category` and `is_deprecated` fields in
the `tag_versions` table.

Adds generic Versionable and VersionFor concerns that encapsulate most
of the history tracking logic. These concerns are designed to make it
easy to add history to any model.

There are a couple notable differences between tag versions and other versions:

* There is no 1 hour edit merge window. All changes to the `category`
  and `is_deprecated` fields produce a new version in the tag history.

* New versions aren't created when a tag is created. Versions are only
  created when a tag is edited for the first time. The tag's initial
  version isn't created until *after* the tag is edited for the first time.

For example, if you change the category of a tag that was last updated
10 years ago, that will create an initial version of the tag backdated
to 10 years ago, plus a new version for your edit.

This is for a few reasons:

* So that we don't have to create new tag versions every time a new tag
  is created. This would be wasteful because most tags never have their
  category or deprecation status change.
* So that if you make a typo tag, your name isn't recorded in the tag's
  history forever.
* So that we can create new tags in various places without having to know
  who created the tag (which may be unknown if the current user isn't set).
* Because we don't know the full history of most tags, so we have to
  deal with incomplete histories anyway.

This has a few important consequences:

* Most tags won't have any tag versions. They only gain tag versions if
  they're edited.
* You can't track /tag_versions to see newly created tags. It only
  shows changes to already existing tags.
* Tag version IDs won't be in strict chronological order. Higher IDs may
  have created_at timestamps before lower IDs. For example, if you
  change the category of a tag that is 10 years old, that will create an
  initial version with a high ID, but with a created_at timestamp dated
  to 10 years ago.

Fixes #4402: Track tag category changes

app/models/artist.rb

@@ -187,7 +187,7 @@ class Artist < ApplicationRecord
       return unless !is_deleted? && name_changed? && tag.present?
 
       if tag.category_name != "Artist" && tag.empty?
-        tag.update!(category: Tag.categories.artist)
+        tag.update!(category: Tag.categories.artist, updater: CurrentUser.user)
       end
     end
   end

app/models/tag.rb

@@ -1,11 +1,15 @@
 # frozen_string_literal: true
 
 class Tag < ApplicationRecord
+  include Versionable
+
   ABBREVIATION_REGEXP = /([a-z0-9])[a-z0-9']*($|[^a-z0-9']+)/
 
   # Tags that are permitted to have unbalanced parentheses, as a special exception to the normal rule that parentheses in tags must balanced.
   PERMITTED_UNBALANCED_TAGS = %w[:) :( ;) ;( >:) >:(]
 
+  attr_accessor :updater
+
   has_one :wiki_page, :foreign_key => "title", :primary_key => "name"
   has_one :artist, :foreign_key => "name", :primary_key => "name"
   has_one :antecedent_alias, -> {active}, :class_name => "TagAlias", :foreign_key => "antecedent_name", :primary_key => "name"
@@ -25,6 +29,8 @@ class Tag < ApplicationRecord
   after_save :update_category_cache, if: :saved_change_to_category?
   after_save :update_category_post_counts, if: :saved_change_to_category?
 
+  versionable :name, :category, :is_deprecated, merge_window: nil, delay_first_version: true
+
   scope :empty, -> { where("tags.post_count <= 0") }
   scope :nonempty, -> { where("tags.post_count > 0") }
   scope :deprecated, -> { where(is_deprecated: true) }
@@ -194,10 +200,11 @@ class Tag < ApplicationRecord
       end
 
       def find_or_create_by_name(name, category: nil, current_user: nil)
-        tag = find_or_create_by(name: normalize_name(name))
+        cat_id = categories.value_for(category)
+        tag = create_with(category: cat_id).find_or_create_by(name: normalize_name(name))
 
-        if category.present? && current_user.present? && Pundit.policy!(current_user, tag).can_change_category?
-          tag.update(category: categories.value_for(category))
+        if category.present? && current_user.present? && cat_id != tag.category && Pundit.policy!(current_user, tag).can_change_category?
+          tag.update(category: cat_id, updater: current_user)
         end
 
         tag

app/models/tag_version.rb

@@ -0,0 +1,7 @@
+# frozen_string_literal: true
+
+class TagVersion < ApplicationRecord
+  include VersionFor
+
+  version_for :tag
+end