Find worst rated mentities

Property definitions

nitc $ StarRatingRepo :: worst_rated
	# Find worst rated mentities
	fun worst_rated: Array[JsonObject] do
		var pipeline = new MongoPipeline
		pipeline.group((new MongoGroup("$mentity")).avg("avg", "$rating"))
		pipeline.sort((new MongoMatch).eq("avg", 1))
		pipeline.limit(10)
		return collection.aggregate(pipeline)
	end
src/doc/api/api_feedback.nit:270,2--277,4