Have OgMembership reference groups via entity reference field

[amitaibu]

Not sure about it, just an idea.

Currently in \Drupal\og\Entity\OgMembership::baseFieldDefinitions we get the reference thank to those fields:

$fields['entity_type'] = BaseFieldDefinition::create('string')
  ->setLabel(t('Group entity type'))
  ->setDescription(t('The entity type of the group.'));

$fields['entity_id'] = BaseFieldDefinition::create('string')
  ->setLabel(t('Group entity id.'))
  ->setDescription(t("The entity ID of the group."));

What if instead of those we had a (configurable) entity reference field?

Pros

• An entity reference better describes what we really do here - an entity reference.
• When we'll build the OgMembership forms for the admin to add users, we'll get the ER field's widget and selection handlers power.

  Cons

• If a site has groups of different entity types, this will require different Og membership types per referenced type. However I think this is going to be an edge case.

/cc @pfrenssen @damiankloip

[amitaibu]

Lets keep as is.