findOrCreateRecord( modelName, id = 1 ) {
		return this.findAll( modelName )
			.then( recordArray => recordArray.content.length
				? recordArray.objectAt( 0 )
				: this.createRecord( modelName, { id } ).save()
			);
	}
Look for an already existing record and avoid automatically generating an empty one.
Instead, create a new one manually if no record was found.
Use Ember's RSVP promises instead of normal promises / async functions.
@param {String} modelName
@param {(Number|String)} id
@returns {Promise.<Model>}