aliases: refactor tag moving code.

* Factor out the code for moving tags from tag aliases to a separate
  TagMover class.

* When aliasing two tags that have conflicting wikis, merge the old wiki
  into the new one instead of failing with an error. Merge the other names
  fields, replace the old wiki body with a message linking to the new
  wiki, and mark the old wiki as deleted.

* When aliasing two tags that have conflicting artist entries, merge the
  old artist into the new one instead of silently ignore the conflict.
  Merge the group name, other names, and urls fields, and mark the old
  artist as deleted.

* When two tags have conflicting wikis or artist entries, but the old
  wiki or artist entry is deleted, then just ignore the old wiki or
  artist and don't try to merge it.

* Fix it so that when saved searches are rewritten, we rewrite negated
  searches too.

app/models/saved_search.rb

@@ -137,6 +137,14 @@ class SavedSearch < ApplicationRecord
         queries = searches.map(&:normalized_query)
         queries.sort.uniq
       end
+
+      def rewrite_queries!(old_name, new_name)
+        has_tag(old_name).find_each do |ss|
+          ss.lock!
+          ss.rewrite_query(old_name, new_name)
+          ss.save!
+        end
+      end
     end
 
     def normalized_query
@@ -146,6 +154,11 @@ class SavedSearch < ApplicationRecord
     def normalize_query
       self.query = PostQueryBuilder.new(query).normalized_query(sort: false).to_s
     end
+
+    def rewrite_query(old_name, new_name)
+      self.query.gsub!(/(?:\A| )([-~])?#{Regexp.escape(old_name)}(?: |\z)/i) { " #{$1}#{new_name} " }
+      self.query.strip!
+    end
   end
 
   attr_reader :disable_labels
@@ -155,6 +168,7 @@ class SavedSearch < ApplicationRecord
   before_validation :normalize_query
   before_validation :normalize_labels
   scope :labeled, ->(label) { where_array_includes_any_lower(:labels, [normalize_label(label)]) }
+  scope :has_tag, ->(name) { where_regex(:query, "(^| )[~-]?#{Regexp.escape(name)}( |$)", flags: "i") }
 
   def validate_count
     if user.saved_searches.count + 1 > user.max_saved_searches