Resolve "Don't run the system test pipeline on every commit"

Closes #1391 (closed).

• 2b06b1ad Auto run CI system tests only on merge

  • Add rule to allow manual run in an MR.
  • Add rule to automatically run after merge.
  • Leave allow_failure to its default value. Specifically leave it to true for manual jobs.
  • Move job to the checks stage so that you can run it immediately, not after the previous stage. #1393 (closed)
  • Remove the system_tests stage because it has no other jobs.
  • Remove the dependencies: [] configuration because there are no more previous stages now.

• f6b6a2bc Do not require system test job to pass