Enhance wording in management pages

Enhance wording in manage_config_email_page.php and
manage_config_work_threshold_page.php pages.

As discussed in issue 0006249, the manage_config_email_page.php is
confusing. One part of the confusion is caused by wording: it should be
'Any user with access level' instead of 'Access Levels'.

The affected string $s_access_levels is also used in other places.
Changing it would work on manage_config_work_threshold_page.php, but
not on manage_config_workflow_page.php, so we need to add a new message
string.

Fixes 0022541