Fix various rubocop issues.

app/models/post.rb

@@ -636,7 +636,7 @@ class Post < ApplicationRecord
     def normalize_tags
       normalized_tags = Tag.scan_tags(tag_string)
       normalized_tags = apply_casesensitive_metatags(normalized_tags)
-      normalized_tags = normalized_tags.map {|tag| tag.downcase}
+      normalized_tags = normalized_tags.map(&:downcase)
       normalized_tags = filter_metatags(normalized_tags)
       normalized_tags = remove_negated_tags(normalized_tags)
       normalized_tags = TagAlias.to_aliased(normalized_tags)
@@ -1256,7 +1256,7 @@ class Post < ApplicationRecord
 
     def children_ids
       if has_children?
-        children.map {|p| p.id}.join(' ')
+        children.map(&:id).join(' ')
       end
     end
   end
@@ -1335,7 +1335,7 @@ class Post < ApplicationRecord
 
       self.is_deleted = false
       self.approver_id = CurrentUser.id
-      flags.each {|x| x.resolve!}
+      flags.each(&:resolve!)
       save
       ModAction.log("undeleted post ##{id}", :post_undelete)
     end