Implement a common_purify for htmLawed and more

We're removing unicode formatting characters as well, such as RTL marks.
For more info on why we're because extra cautious (but may accept the
characters in later versions) you can read:
https://blog.malwarebytes.org/online-security/2014/01/the-rtlo-method/

lib/util.php

@@ -576,6 +576,25 @@ function common_canonical_email($email)
     return $email;
 }
 
+function common_purify($html)
+{
+    require_once INSTALLDIR.'/extlib/htmLawed/htmLawed.php';
+
+    $config = array('safe' => 1,
+                    'deny_attribute' => 'id,style,on*');
+
+    $html = common_remove_unicode_formatting($html);
+
+    return htmLawed($html, $config);
+}
+
+function common_remove_unicode_formatting($text)
+{
+    // Strip Unicode text formatting/direction codes
+    // this is pretty dangerous for visualisation of text and can be used for mischief
+    return preg_replace('/[\\x{200b}-\\x{200f}\\x{202a}-\\x{202e}]/u', '', $text);
+}
+
 /**
  * Partial notice markup rendering step: build links to !group references.
  *
@@ -585,9 +604,9 @@ function common_canonical_email($email)
  */
 function common_render_content($text, Notice $notice)
 {
-    $r = common_render_text($text);
-    $r = common_linkify_mentions($r, $notice);
-    return $r;
+    $text = common_render_text($text);
+    $text = common_linkify_mentions($text, $notice);
+    return $text;
 }
 
 /**
@@ -829,14 +848,15 @@ function common_find_mentions_raw($text)
 
 function common_render_text($text)
 {
-    $r = nl2br(htmlspecialchars($text));
+    $text = common_remove_unicode_formatting($text);
+    $text = nl2br(htmlspecialchars($text));
 
-    $r = preg_replace('/[\x{0}-\x{8}\x{b}-\x{c}\x{e}-\x{19}]/', '', $r);
-    $r = common_replace_urls_callback($r, 'common_linkify');
-    $r = preg_replace_callback('/(^|\&quot\;|\'|\(|\[|\{|\s+)#([\pL\pN_\-\.]{1,64})/u',
-                function ($m) { return "{$m[1]}#".common_tag_link($m[2]); }, $r);
+    $text = preg_replace('/[\x{0}-\x{8}\x{b}-\x{c}\x{e}-\x{19}]/', '', $text);
+    $text = common_replace_urls_callback($text, 'common_linkify');
+    $text = preg_replace_callback('/(^|\&quot\;|\'|\(|\[|\{|\s+)#([\pL\pN_\-\.]{1,64})/u',
+                function ($m) { return "{$m[1]}#".common_tag_link($m[2]); }, $text);
     // XXX: machine tags
-    return $r;
+    return $text;
 }
 
 /**