Port the code to PHP 8.5.

Replace the short open tags and the removed functions (mcrypt, mysql_error,
salt-less crypt). Decode the reCAPTCHA reply as JSON, turn off mysqli
exceptions, drop stripslashes(), and default missing input values.
Fix the misplaced continue in prune_images().

sidebar.inc.php

@@ -1,6 +1,6 @@
 <aside id="sidebar">
   <div class="logo">
-    <a <?
+    <a <?php
 // Match files that use the main x.x.x logo
 if (preg_match("#^(/$|/index|/article)#", $_SERVER['REQUEST_URI'])) {
     echo 'class="mono" ';
@@ -27,7 +27,7 @@ if (preg_match("#^(/$|/index|/article)#", $_SERVER['REQUEST_URI'])) {
       <li><a href="/about">About <span class="xxx">x&middot;x&middot;x</span></a></li>
       <li><a href="/article?start=5">Archive</a></li>
       <li><a href="/publish">Add</a>/<a href="/publish/edit" class="sidelink">Edit</a></li>
-<? if (isset($is_admin) && $is_admin) {
+<?php if (isset($is_admin) && $is_admin) {
         echo '      <li><a href="/publish/addeditor">Add editor</a></li>'."\n";
         echo '      <li><a href="/publish/logout">Logout</a></li>'."\n";
 }