1521 disable life event form delete if still used

[gchi25]

PR Summary

This work ensures life event form content can not be delete if still used.
If a life event form still used, it lists the contents that use this this life event form and disables the delete confirmation button.

Related Github Issue

• Fixes Cannot delete life event form if still used #1521

Detailed Testing steps

To test in local development site or in dev site.

• Pull changes locally
• Make local development site up at http://localhost
• Navigate to admin/content?combine=&type=bears_life_event_form&status=All&langcode=All
• Go to life event form "Benefit finder: death of a loved one" edit page
• Click Delete
• Verify the warning message "cannot delete the life event form" and list of life event forms
• verify that no Delete confirmation button

• Go to life event form "Benefit finder: retirement" edit page
• Remove life event form "Benefit finder: death of a loved one" in relevant benefit

• Click Save
• Go to life event form "Benefit finder: disability" edit page
• Remove life event form "Benefit finder: death of a loved one" in relevant benefit
• Click Save

Now delete life event form "Benefit finder: death of a loved one" again

• Go to life event form "Benefit finder: death of a loved one" edit page
• Click Delete
• Verify message "This action cannot be undone"
• Verify message "The following entity references will be skipped and leave orphans" and list of benefits

• Click Delete button to delete this life event form
• Verify life event form "Benefit finder: death of a loved one" deleted

[diegocob]

Went through the QA steps and it's working as expected.