Don't show list people in the "add watcher" dialog that cannot watch the issue

It is possible for the "Add Watcher" dialog on the view issue page to suggest a user that cannot watch the issue. The client will render an error saying the user cannot be added. JIRA should not show users that cannot be added.

1. Restore JIRA QA data.
2. Goto an issue XSS-21.
3. Try and add "<script>alert(document.cookie)</script>" as watcher.
4. (BUG) You will get an error saying that the user cannot see the issue.