Don't grant SSH privileges to inactive users [MNG-918]

Created by: bradenmacdonald

Our code to grant SSH access was mistakenly including user accounts that were deactivated. This is a small fix to exclude them. I have already updated accounts in the DB so that even without this fix, they aren't included.

Testing instructions: deactivate a user via the django admin, then use the django shell to query an appserver's admin_users list. Or just run the before + after versions of this ORM statement (users = UserProfile.objects. ... .values_list('github_username', flat=True)) in the django shell. Confirm that deactivated users aren't in the list.