Modernize, implement HTTP auth

[daniel-beck]

Demonstrates how one could implement an authentication challenge for HTTP Git repos, addressing jenkinsci/git-server-plugin#112 (comment):

my distant memory of it is that HTTP mode is essentially useless because it does not support authentication at all and so is only appropriate for an unsecured controller

Minimal testing and requires the linked PR for a git clone, would not consider this to be production code. Probably better off integrated properly in git-server.