Fix return value of email_bug_reminder()

The function now returns an array containing only the users to whom the
reminder e-mail was actually sent (i.e. excludes failures, blank e-mails,
etc).

In addition, the return value is now an array of user id's instead of
user names, which indeed makes more sense from an API perspective, as
mentioned by Julian Fitzell in 5ea3d0bc837ef14fd66b63d63e5481ea3addcf29,
since the function's only caller is bug_reminder.php which exclusively
relies on user id's.

Fixes 0015472