feat: surface Today issue blockers (#435)

frontend/index.html

@@ -262,6 +262,11 @@
     <button class="issue-retry" id="retry-issue-load" type="button" hidden>Reload latest issue</button>
     <div class="row"><span id="issue-labels"></span><span class="small" id="issue-assignees"></span></div>
     <div class="issue-sheet-content markdown-content" id="issue-sheet-body"></div>
+    <section class="issue-blockers" id="issue-blockers" aria-labelledby="issue-blockers-title" hidden>
+      <h2 id="issue-blockers-title">Blocked by</h2>
+      <div class="issue-blocker-list" id="issue-blocker-list"></div>
+      <div id="issue-blocker-status" class="small" aria-live="polite"></div>
+    </section>
     <h2>Full conversation</h2>
     <div id="issue-comments"></div>
     <button class="conversation-more" id="load-older-issue-comments" type="button" hidden>Load older messages</button>