Fix page_description helper performance

This helper is used for extracting part of the issue / MR / whatever
description for use in the description meta tag:

1. To do that, we look at the source of the Markdown description.
2. We then strip out all HTML tags.
3. And then take the first 30 words.

Doing that can be really slow - especially as Markdown is supposed to be
treated as plain text. There are many better ways to do this, but the
immediate performance fix is to swap steps 2 and 3. This does mean that
the description may be less than 30 words (or even empty), but it is
much faster when the description is very long.

app/helpers/page_layout_helper.rb

+# coding: utf-8
 # frozen_string_literal: true
 
 module PageLayoutHelper
@@ -36,7 +37,7 @@ module PageLayoutHelper
     if description.present?
       @page_description = description.squish
     elsif @page_description.present?
-      sanitize(@page_description, tags: []).truncate_words(30)
+      sanitize(@page_description.truncate_words(30), tags: [])
     end
   end
 

changelogs/unreleased/62116-performance-issue-502-errors-on-rendering-of-issues-with-heavy-markdown-contents.yml

+---
+title: Fix performance issue with large Markdown content in issue or merge request
+  description
+merge_request: 28597
+author:
+type: performance

spec/helpers/page_layout_helper_spec.rb

@@ -38,6 +38,14 @@ describe PageLayoutHelper do
 
       expect(helper.page_description).to eq 'Bold Header'
     end
+
+    it 'truncates before sanitizing' do
+      helper.page_description('<b>Bold</b> <img> <img> <img> <h1>Header</h1> ' * 10)
+
+      # 12 words because <img> was counted as a word
+      expect(helper.page_description)
+        .to eq('Bold    Header Bold    Header Bold    Header Bold    Header Bold    Header Bold    Header...')
+    end
   end
 
   describe 'page_image' do