additional changes applied to the previous commit:
- moved the database table rename code to the 'rename_sql_db_tables'
function (since NC20).
- made sure that the 'rename_sql_db_tables' function is called after
a new Nextcloud installation (since NC20).
- removed --database-table-prefix command line switch from
"occ maintenance:install ..." command for a Nextcloud installation.
- suppressed output of occ-command errors by default.
- removed explicit "occ trashbin:expire" cron job