Fix link to mariadb explain analyzer

- avoid double urlencoding of the query
- properly escape html entities

Signed-off-by: Michal Čihař <michal@cihar.com>

libraries/Util.php

@@ -1220,10 +1220,10 @@ class Util
                         ) . ']';
                     $url = 'https://mariadb.org/explain_analyzer/analyze/'
                         . '?client=phpMyAdmin&raw_explain='
-                        . urlencode(self::_generateRowQueryOutput($sql_query));
+                        . self::_generateRowQueryOutput($sql_query);
                     $explain_link .= ' ['
                         . self::linkOrButton(
-                            'url.php?url=' . urlencode($url),
+                            htmlspecialchars('url.php?url=' . urlencode($url)),
                             sprintf(__('Analyze Explain at %s'), 'mariadb.org'),
                             array(),
                             true,