add userboxes and use them instead of flash

views/threads/thread.etlua

@@ -1,4 +1,5 @@
 <% render("views.common.topnav") -%>
+<% local is_locked = ntob(thread.is_locked) %>
 <main>
   <nav class="darkbg">
     <h1 class="thread-title"><%= thread.title %></h1>
@@ -13,10 +14,13 @@
   <% render("views.common.pagination", {page_count = pages, current_page = page}) %>
 </nav>
 
-<% if not me:is_guest() then %>
+<% if is_locked then -%>
+  <% render("views.common.infobox", {kind = constants.InfoboxKind.LOCK, msg = "This thread is locked."}) %>
+<% end -%>
+<% if not me:is_guest() and not is_locked then %>
 <h1>Respond to "<%= thread.title %>"</h1>
 <form method="post">
   <textarea id="post_content" name="post_content" placeholder="Response body" required></textarea><br>
-  <input type="submit" value="Reply">
+  <input type="submit" value="Post reply">
 </form>
 <% end %>