Manually give attendees points

[RitamNandi]

• Admins should be able to update an attendee's points manually on the admin site (such as if a special event requires it)
• Can only add nonnegative integer amounts. I guess if we want to be able to take away points using this then we should change that, but I don't see why we should be taking away someone's points.
• Added tests for this as well

If we want to be able to take away points, we can just update the validator, and add a check to make sure the person's points don't go negative.

[Bahl-Aryan]

LGTM