aliases/implications: change automatic retirement rules.

Change the rules for automatically retiring aliases and implications:

* Retire aliases to tags that are empty, or that are for a general or
  artist tag that hasn't received any new posts in the last two years.
* Retire implications from tags that are empty.
* Don't retire aliases or implications for character, copyright, or
  meta tags any more, unless the tags are empty.

app/models/tag_alias.rb

@@ -6,6 +6,8 @@ class TagAlias < TagRelationship
 
   before_create :delete_conflicting_relationships
 
+  scope :empty, -> { joins(:consequent_tag).merge(Tag.empty) }
+
   def self.to_aliased(names)
     names = Array(names).map(&:to_s)
     return [] if names.empty?

app/models/tag_implication.rb

@@ -15,6 +15,8 @@ class TagImplication < TagRelationship
   validate :meets_tag_size_requirements, on: :request
   validate :has_wiki_page, on: :request
 
+  scope :empty, -> { joins(:antecedent_tag).merge(Tag.empty) }
+
   concerning :HierarchyMethods do
     class_methods do
       def ancestors_of(names)

app/models/tag_relationship.rb

@@ -16,6 +16,12 @@ class TagRelationship < ApplicationRecord
   scope :deleted, -> {where(status: "deleted")}
   scope :retired, -> {where(status: "retired")}
 
+  # TagAlias.artist, TagAlias.general, TagAlias.character, TagAlias.copyright, TagAlias.meta
+  # TagImplication.artist, TagImplication.general, TagImplication.character, TagImplication.copyright, TagImplication.meta
+  TagCategory.categories.each do |category|
+    scope category, -> { joins(:consequent_tag).where(consequent_tag: { category: TagCategory.mapping[category] }) }
+  end
+
   before_validation :normalize_names
   validates :status, inclusion: { in: STATUSES }
   validates :antecedent_name, presence: true
@@ -103,10 +109,6 @@ class TagRelationship < ApplicationRecord
       # "TagAlias" -> "tag alias", "TagImplication" -> "tag implication"
       self.class.name.underscore.tr("_", " ")
     end
-
-    def retirement_message
-      "The #{relationship} [[#{antecedent_name}]] -> [[#{consequent_name}]] has been retired."
-    end
   end
 
   def antecedent_and_consequent_are_different