Deactivated user is allowed to be added as watcher

If a user is deactivated, which means it belongs to no existing group in JIRA, it is still able to be set as an issue watcher if it has the Browser Project Permission.

Steps to replicate:

1. deactivate an user.
2. ensure it has Browse Project permission.
3. try to add it as watcher in any existing issue on the project it has the permission to browse.