Don't reload ActiveRecord objects when building note URLs

When we build a note URL, and we have the note loaded already, there are two
cases:

1. The `noteable` is already loaded. In that case, this is faster as it doesn't
   build a new AR object from the query.
2. The `noteable` is not already loaded. In that case, this change is no worse
   than the previous code.

changelogs/unreleased/fix-n-plus-one-in-url-builder.yml

+---
+title: Improve issue rendering performance with lots of notes from other users
+merge_request:
+author:

lib/gitlab/url_builder.rb

@@ -52,15 +52,13 @@ module Gitlab
         commit_url(id: object.commit_id, anchor: dom_id(object))
 
       elsif object.for_issue?
-        issue = Issue.find(object.noteable_id)
-        issue_url(issue, anchor: dom_id(object))
+        issue_url(object.noteable, anchor: dom_id(object))
 
       elsif object.for_merge_request?
-        merge_request = MergeRequest.find(object.noteable_id)
-        merge_request_url(merge_request, anchor: dom_id(object))
+        merge_request_url(object.noteable, anchor: dom_id(object))
 
       elsif object.for_snippet?
-        snippet = Snippet.find(object.noteable_id)
+        snippet = object.noteable
 
         if snippet.is_a?(PersonalSnippet)
           snippet_url(snippet, anchor: dom_id(object))