Relation widget: cascade deletes

[r1b]

- Do you want to request a feature or report a bug?

Feature

- What is the current behavior?

Deleted objects do not cascade to their references in relation widgets

- If the current behavior is a bug, please provide the steps to reproduce.

1. Create an object A
2. Create an object B with a relation that refers to A
3. Delete A
4. An orphan remains in object B. It may not be displayed visually but it is still in the raw data.

- What is the expected behavior?

The data referring to the now-deleted object should be removed

- Please mention your CMS, node.js, and operating system version.

Netlify Version: 0.5.0
Node Version: 8.6.0
OS Version: Variable

[erquhart]

Relation dependency management is definitely on our radar - we've taken a swing at it in the past, but nothing landed. It would almost definitely require tracking relations via metadata.

[erquhart]

Closing in favor of #192, which captures the larger relation dependency story.