allow use.gist.user.home with gist service login

[gordonwoodhull]

I wanted single user regular login with rcloud-gist-services for a demo instance, because we don't know how to do AD in Azure yet.

I got it to work with

exec.auth: attESSec
github.auth: exec.token
github.client.id: default
github.client.secret: X
rational.githubgist: true
github.api.url: http://rc-meta:13020/
github.auth.forward: http://rcloud-4.research.att.com/login_successful.R
use.gist.user.home: yes
...

However, `use.gist.user.home` did not work because of `exec.auth`, so I commented out part of the check in `RC.authenticate`:

```R
  if (# !nzConf("exec.auth") &&
     nzConf("use.gist.user.home") && length(grep("yes", getConf("use.gist.user.home")))) {

It might be nice to support this scenario somehow. (Although if things go well and we move to AD, this won't be needed.)