Add "create multiple users" feature

[amaurymedeiros]

Add add_multiple method to Users controller. It should receive either a list of users or a .csv file.

Suggested tests:
Add multiple users using an ADMIN user (should return 200 - OK)
Add multiple users using a user from any other level (should return 401 - Unauthorized)
Add multiple users from a list that contains an already existing user (should return 500)
Add multiple users with a bad formatted list/file (should return 500)