Validate Ghe Connectivity in getSettings()

[npearce]

Verify that we have the right settings in the webhook server, perform an access check to the BIG-IP's repo.

Maybe use octokit.repo.getShaOfCommitRef(owner, repo, ref) to ensure access to the repo?

This will execute every time the settings are changed, and every time a webhook message is received, so we can fail, and createIssue() if no access to repo.

Requires #35

[npearce]

Implemented and tested.