Made username validation configurable through AUTH_USERNAME_VALIDATORS setting

[Shekhar Gyanwali]

Hi Everyone,

First time contributor here, I have submitted PR 13114 for ticket 27807 to allow customization of username validation through global setting similar to password validation.

 I would like to gather some feedback on my implementation/approach. I have added a new page under topics/auth for Username Validation to describe the solution(similar to password validation page).

Thanks to Carlton and Will, Django Podcast encouraged me to start looking into opensource contribution and to Jacob Walls who helped me a lot with the review.

Regards

Shekhar Gyanwali