Bug 1398889 - Add param 'silent_users' that never trigger sending bugmail

Bug 1398889 - Add param 'silent_users' that never trigger sending bugmail

* Add param silent_users
* Add method is_silent_user to User class, which returns true if the login name matches an entry in silent_users.
* Change user_wants_mail() to return false if the changer is a silent user.
* Change is_global_watcher() in User class code to use any instead of grep
* Change regex used to parse param 'globalwatchers' to use consistent regex in BugMail.pm and User.pm