feat: load update conversations progressively (Closes #761)

frontend/index.html

@@ -763,6 +763,7 @@
     <button class="update-new-activity-jump" id="jump-update-new-activity" type="button" hidden>Jump to new activity</button>
     <button class="conversation-more" id="load-older-update-comments" type="button" hidden>Load older messages</button>
     <div id="update-conversation-status" class="small" aria-live="assertive"></div>
+    <button class="update-retry" id="retry-update-conversation" type="button" hidden>Retry conversation</button>
     <details>
       <summary><h2>Subject context</h2></summary>
       <div class="update-sheet-content muted markdown-content" id="update-subject-body"></div>