Identify duplicated email addresses

When email_ensure_unique = ON, it is an error to have multiple user
accounts with the same email address in the database, which can happen
when this config's value is changed from OFF to ON.

This helps the admin identify offending accounts by displaying

• a warning sign next to the email address on Manage Accounts page
• warning + info message on Account Edit pages (both account_page.php
  and manage_user_edit_page.php)

Fixes 0032787