CRM-13625 fix - Twitter text references thankyou page with session, not ...

As per the usage of url parameter for tweeting, it should pass url-encoded link otherwise as in current situation it carries the current page url instead. For the same reason in our case it replaces with Event Thankyou page url. 
          Still a minor issue persist  after changes as urlencoded & is not transformed back to & but &amp; instead. Also replacing it with original symbol in url-encoded value is not of much help. 

Reference: https://dev.twitter.com/docs/intents