relocate db methods into their own trait, re #1265

db/migrations/5.1.21_missing_indices_v50.php

db/migrations/5.1.24_biest_348.php

db/migrations/5.1.27_add_index_to_cw_user_progresses_v50.php

...igrations/5.1.29_add_index_resource_booking_intervals.php

db/migrations/5.1.30_remove_column_termine_topic_id.php

db/migrations/5.1.33_remove_column_ex_termine_topic_id.php

lib/migrations/DatabaseMigrationTrait.php