Allow optional bidirectional relationships

Changing the issue title to better reflect the work being done

Here is the patch updated for 5.0.0-beta3 and the reroll diff (only some very light changes in the patch due to #3075276: Adding option to prepend referenced entities if desired → creating some offset).

Switching to needs review.

After applying the patch, one of my CERs wasn't working because it is now treated as uni directional. Setting it to bi directional fixed it (and the complete sync patch was helpful too!).

I think we should assume bi-directional by default, or do an update hook to set existing config values.

This patch has been working for me for a few months. I think we just need an update hook that sets the value to true in existing config.

Was there another way to have uni directional relationships before this?