Fix #5013: BUR model doesn't validate tags.

Don't allow users to request aliases, implications, or renames for invalid tag names.

As a side effect, it's no longer possible to request shortcut aliases like
`/hr -> hakurei_reimu` (slash abbreviations still exist, but they can't
be overridden with aliases). Tests involving these types of aliases are
removed.

app/logical/bulk_update_request_processor.rb

@@ -112,11 +112,15 @@ class BulkUpdateRequestProcessor
           end
 
         when :rename
-          tag = Tag.find_by_name(args[0])
-          if tag.nil?
+          old_tag = Tag.find_by_name(args[0])
+          new_tag = Tag.find_by_name(args[1]) || Tag.new(name: args[1])
+
+          if old_tag.nil?
             errors.add(:base, "Can't rename #{args[0]} -> #{args[1]} (the '#{args[0]}' tag doesn't exist)")
-          elsif tag.post_count > MAXIMUM_RENAME_COUNT
+          elsif old_tag.post_count > MAXIMUM_RENAME_COUNT
             errors.add(:base, "Can't rename #{args[0]} -> #{args[1]} ('#{args[0]}' has more than #{MAXIMUM_RENAME_COUNT} posts, use an alias instead)")
+          elsif new_tag.invalid?(:name)
+            errors.add(:base, "Can't rename #{args[0]} -> #{args[1]} (#{new_tag.errors.full_messages.join("; ")})")
           end
 
         when :mass_update

app/models/tag_relationship.rb

@@ -26,8 +26,8 @@ class TagRelationship < ApplicationRecord
 
   before_validation :normalize_names
   validates :status, inclusion: { in: STATUSES }
-  validates :antecedent_name, presence: true
-  validates :consequent_name, presence: true
+  validates :antecedent_name, presence: true, tag_name: true, if: :antecedent_name_changed?
+  validates :consequent_name, presence: true, tag_name: true, if: :consequent_name_changed?
   validates :approver, presence: { message: "must exist" }, if: -> { approver_id.present? }
   validates :forum_topic, presence: { message: "must exist" }, if: -> { forum_topic_id.present? }
   validate :antecedent_and_consequent_are_different