Added the form and list for mail filtering rules in the interface.

interface/web/mail/form/mail_user.tform.php

interface/web/mail/form/mail_user_filter.tform.php

interface/web/mail/lib/lang/en_mail_user.lng

interface/web/mail/lib/lang/en_mail_user_filter.lng

interface/web/mail/lib/lang/en_mail_user_filter_list.lng

interface/web/mail/list/mail_user_filter.list.php

interface/web/mail/mail_user_filter_del.php

interface/web/mail/mail_user_filter_edit.php

interface/web/mail/templates/mail_user_custom_rules_edit.htm

interface/web/mail/templates/mail_user_filter_edit.htm

interface/web/mail/templates/mail_user_filter_list.htm

interface/web/mail/templates/mail_user_mailfilter_edit.htm