[FIXES JENKINS-47406] Milestone JOB_LOADED is now attained after cleaning up obsolete items (#3078)

* [FIXES JENKINS-47406] Milestone JOB_LOADED is now attained after cleaning up obsolete items

This allows @Initializer(after=InitMilestone.JOB_LOADED) implementations
to create items safely without risking them to be removed.

* [JENKINS-47406] Add test with an initializer creating a job

* Revert "[FIXES JENKINS-47406] Milestone JOB_LOADED is now attained after cleaning up obsolete items"

This reverts commit 79ce796b00fde11fda586c9c9e2b8e93a53e2b31.

* [JENKINS-47406] Reinstate the test with Jesse's solution

* Reverted this commit by mistake

Revert "Revert "[FIXES JENKINS-47406] Milestone JOB_LOADED is now attained after cleaning up obsolete items""

This reverts commit 36c499c58c83427c7b8a6d3cc5902ec26732b2c7.

* Add link to hpi sources

* Fix Mix-up after the previous merge