Refactor sandmark-nightly

[shakthimaan]

Tasks

• Move results to nightly/ folder.
• Remove top-level files: sandmark-nightly-client-crontab, start_app.sh, start_docker.sh.
• Remove logs folder.
• Generate daily cron log in cron-YYYY-MM-DD.log filename format.
• Move scripts/nigthly_* to Sandmark. Delete other scripts in this folder.
• Check jq correctly parses custom.json file.
• Always clone fresh copy of Sandmark for each custom entry.
• Link to Sandmark README in sandmark-nightly-config.
• Do ls _results in output of each run.
• Backport notebooks/ to Sandmark and remove the folder.
  Update sandmark notebooks with latest changes from sandmark-nightly sandmark#259
  Update notebooks/ to 5.00.0+trunk sandmark#279