DEV: Update username in new quote format - Part 1 (#22032)

When we introduced the new quote format with full-name display name:

```
[quote="Ted Johansson, post:1, topic:2, username:ted"]
we overlooked the code responsible for rewriting quotes when a user's name is changed.
```

The functional part of this change adds support for the new quote format in the code that updates quotes when a user's username changes. See the test case in `spec/services/username_changer_spec.rb` for the details.

In addition, this change adds a regression test for PrettyText to cover the new quote format, and extracts the code responsible for rewriting raw and cooked quotes into its own `QuoteRewriter` class. The functionality of the latter is tested through the tests in `spec/services/username_changer_spec.rb`.

spec/lib/pretty_text_spec.rb

@@ -28,6 +28,28 @@ RSpec.describe PrettyText do
 
       before { User.stubs(:default_template).returns(default_avatar) }
 
+      it "correctly extracts usernames from the new quote format" do
+        topic = Fabricate(:topic, title: "this is a test topic :slight_smile:")
+        expected = <<~HTML
+          <aside class="quote no-group" data-username="codinghorror" data-post="2" data-topic="#{topic.id}">
+          <div class="title">
+          <div class="quote-controls"></div>
+          <a href="http://test.localhost/t/this-is-a-test-topic/#{topic.id}/2">This is a test topic <img width="20" height="20" src="/images/emoji/twitter/slight_smile.png?v=#{Emoji::EMOJI_VERSION}" title="slight_smile" loading="lazy" alt="slight_smile" class="emoji"></a>
+          </div>
+          <blockquote>
+          <p>ddd</p>
+          </blockquote>
+          </aside>
+        HTML
+
+        expect(
+          cook(
+            "[quote=\"Jeff, post:2, topic:#{topic.id}, username:codinghorror\"]\nddd\n[/quote]",
+            topic_id: 1,
+          ),
+        ).to eq(n(expected))
+      end
+
       it "do off topic quoting with emoji unescape" do
         topic = Fabricate(:topic, title: "this is a test topic :slight_smile:")
         expected = <<~HTML