Refactor redirect flow and fix URLs for --bind-addr

[klizhentas]

No description provided.

[osterman]

Thanks @klizhentas !

[r0mant]

Unfinished comment.

[russjones]

Code looks fine, but it's large enough where it can be encapsulated within it's own struct with methods that you can call from SSHAgentSSOLogin.

Something like:

type callbackServer struct{
}

func (s *callbackServer) handleCallback(w http.ResponseWriter, r *http.Request) {
   [...]
}

func (s *callbackServer) handleRedir(w http.ResponseWriter, r *http.Request) {
   [...]
}

func (s *callbackServer) Start() {
   [...]
}

func (s *callbackServer) Close() error {
   [...]
}

[klizhentas]

@russjones I'll look into refactor a bit later, may be do some work in master