Fix performance issue in manage_user_page.php

@atrol reported the problem in issue 0032787:68077, as the page took
approximately 3 seconds to load, vs 0.2 seconds before commit
d668192806a7457552229b1900f10d5e89ba6754.

Code now relies on user_get_duplicate_emails() to globally identify the
user accounts with duplicate e-mail addresses, instead of 50 individual
calls to user_get_duplicate_emails().

With this, the page loads under 0.5 seconds on mantisbt.org bugtracker,
vs 0000007:0000003 seconds before this change. This is still about twice as slow as
before d668192806a7457552229b1900f10d5e89ba6754, but I believe that's
acceptable for an admin page.

Fixes 0032787