proposal Fix #6692 and #2962 add backup interval setting for databases

interface/lib/classes/remote.d/sites.inc.php

interface/lib/classes/sites_database_plugin.inc.php

interface/web/sites/form/database.tform.php

interface/web/sites/lib/lang/ar_database.lng

interface/web/sites/lib/lang/bg_database.lng

interface/web/sites/lib/lang/br_database.lng

interface/web/sites/lib/lang/ca_database.lng

interface/web/sites/lib/lang/cn_database.lng

interface/web/sites/lib/lang/cz_database.lng

interface/web/sites/lib/lang/de_database.lng

interface/web/sites/lib/lang/dk_database.lng

interface/web/sites/lib/lang/el_database.lng

interface/web/sites/lib/lang/en_database.lng

interface/web/sites/lib/lang/es_database.lng

interface/web/sites/lib/lang/fi_database.lng

interface/web/sites/lib/lang/fr_database.lng

interface/web/sites/lib/lang/hr_database.lng

interface/web/sites/lib/lang/hu_database.lng

interface/web/sites/lib/lang/id_database.lng

interface/web/sites/lib/lang/it_database.lng