Display pending email verification information

On account_page.php, manage_user_page.php and manage_user_edit_page.php,
we display a warning message to inform the user that the email address
is pending user validation.

Adds a new print_email_pending_verification_warning() function similar
to print_email_not_unique_warning().

To efficiently process the list of user accounts in manage_user_page.php
a new token_get_by_type() function allows retrieving a list of all
tokens of a given type.

Issue 0036005