Find worst rated mentities

Property definitions

nitc $ StarRatingRepo :: users_ratings
	# Find worst rated mentities
	fun users_ratings: Array[JsonObject] do
		var pipeline = new MongoPipeline
		pipeline.group((new MongoGroup("$user")).sum("count", 1))
		pipeline.sort((new MongoMatch).eq("count", -1))
		pipeline.limit(10)
		return collection.aggregate(pipeline)
	end
src/doc/api/api_feedback.nit:279,2--286,4