Feature: Firestore subcollections

[brettwillis]

Allow a model to be configured as a subcollection of a parent model, by adding a parentModel option.

{
  "options": {
    "parentModel": "users"
  },
  "attributes": {
    "owner": {
      "model": "users"
    }
  }
}

If there is a one-way relation to the parent model, then that reltion will become the partent document of the subcollection. If there is no one-way relation to the parent model, then one will be implicitly created with the name "parent".

If the parent relation is changed, then the document will be deleted from the old subcollection and added to the new subcollection.