tags: ensure aliased tag categories stay in sync.

* When a tag's category is changed, also change the category of any aliases pointing to it. For
  example, if "ff7" is aliased to "final_fantasy_vii", and "final_fantasy_vii" is changed to a
  copyright tag, then change the empty "ff7" tag to be a copyright tag too.

* Don't allow changing the category of an aliased tag. For example, if "ff7" is aliased to
  "final_fantasy_vii", then don't allow changing the "ff7" tag to be a non-copyright tag.

This ensures that the categories of aliased tags stay in sync with that of their parent tags. This
way aliased tags are colored correctly in wikis and other places.

app/models/tag.rb

@@ -23,8 +23,10 @@ class Tag < ApplicationRecord
   validates :name, tag_name: true, uniqueness: true, on: :create
   validates :name, tag_name: true, on: :name
   validates :category, inclusion: { in: TagCategory.category_ids }
+  validate :validate_category, if: :category_changed?
 
   before_create :create_character_tag_for_cosplay_tag, if: :is_cosplay_tag?
+  after_save :update_tag_alias_categories, if: :saved_change_to_category?
   after_save :update_category_cache, if: :saved_change_to_category?
   after_save :update_category_post_counts, if: :saved_change_to_category?
 
@@ -173,6 +175,21 @@ class Tag < ApplicationRecord
     def update_category_cache
       Cache.put("tc:#{Cache.hash(name)}", category, 3.hours)
     end
+
+    # When a tag's category is changed, also change the categories of any aliases pointing to it.
+    def update_tag_alias_categories
+      consequent_aliases.each do |tag_alias|
+        if tag_alias.antecedent_tag.category != category
+          tag_alias.antecedent_tag.update!(category: category, updater: updater)
+        end
+      end
+    end
+
+    def validate_category
+      if is_aliased? && category != aliased_tag.category
+        errors.add(:base, "Can't change the category of an aliased tag")
+      end
+    end
   end
 
   concerning :NameMethods do