Add Facebook to robot block list

https://github.com/PrivateBin/PrivateBin/issues/156
https://www.facebook.com/externalhit_uatext.php

.htaccess.disabled

@@ -1,3 +1,3 @@
 RewriteEngine on
-RewriteCond %{HTTP_USER_AGENT} ^.*(bot|spider|crawl|https?://|WhatsApp|SkypeUriPreview) [NC]
+RewriteCond %{HTTP_USER_AGENT} ^.*(bot|spider|crawl|https?://|WhatsApp|SkypeUriPreview|facebookexternalhit) [NC]
 RewriteRule .* - [R=403,L]