Enable logined user the ability to subscribe bugs

Before I got an account I could use the subscribe to keep myself been notified if anything on the bug,
but after I logged in I can't subscribe any bugs, but I'm not a php group member. so any update
can  not  reach me.

This patch shows the subscribe & unsubscribe button. 

even an php group member can subscribe it , since the email will send directly to him.
if he use filter, that might been helpful too.


BTW: 
The auth_user->registered didn't exists at all:
https://github.com/php/web-bugs/blob/master/include/functions.php#L133
so I remove it: https://github.com/reeze/web-bugs/pull/new/add-dev-subscrib#L0L72