entity lifecycle management: handling deletion of referenced entities

1. Trigger and Action entities may be referenced in Rules.
2. Actions may by referenced by Sequences.

What should happen when a referenced entity is deleted (or an entitlement change occurs to a referenced entity)?

Options include:
- Delete the referencing entity (e.g., delete Rule).
- Disable the referencing entity (e.g., disable Rule).
- Don't allow deleting the referenced entity (e.g., the user cannot delete Trigger/Action until they delete Rule).

A rule could be automatically disabled if any of the following occur:
- A referenced trigger or action is deleted (or generally "disabled").
- The owner of the rule loses activate privilege on the referenced trigger.
- The owner of the rule loses activate privilege on the referenced action.

Similarly, a sequence could be automatically disabled if any of the following occur:
- A referenced action is deleted (or generally "disabled").
- The owner of the sequence loses activate privilege on the referenced action.

Since actions may be a sequence, where applicable, a recursive check may be necessary.