Properly sanitize everything in the XML we send to the XMPP server

in this order:
- Make sure it is utf-8 encoded
- Remove all chars that are invalid in XML
- Escape all XML special chars (&'"<>)

src/xmpp/xmpp_stanza.hpp

@@ -7,6 +7,7 @@
 
 std::string xml_escape(const std::string& data);
 std::string xml_unescape(const std::string& data);
+std::string sanitize(const std::string& data);
 
 /**
  * Represent an XML node. It has