Create users summary endpoint

[ThukuWakogi]

Create an endpoint through which we one can get a summary on users
Add data to JobGrade as provided in the Competency Evaluation Framework and add jobgrades to several users
using the data generated give a summary of the users indicating the total amount of users plus the number of users per job grade
The endpoint should return a json in the following format

{
	"total_number_of_user": 132,
	"champions": 11,
	"Experts": 22,
	"Practitioners": 35,
	"Beginners": 64
}

The endpoint url should be /api/users/summary/