delete files on user delete (#1452)
delete files on user delete (#1452)

fixes #1451
(I did not do a cascade delete for 2 reasons: Sqlite does not support the migration easily to alter a foreign key AND files would be orphaned in the storage with no record in the db)
btcpayserver/btcpayserver,btcpayserver/btcpayserver,btcpayserver/btcpayserver,btcpayserver/btcpayserver