Verify access token validity and scopes on startup

[sorenlouv]

Currently if the access token is invalid or doesn't have the correct scopes, the application will crash with this error:

Error: Command failed: git push johndoe backport/7.x/pr-12345:backport/7.x/pr-12345 --force
remote: Permission to johndoe/kibana.git denied to johndoe.
fatal: unable to access 'https://jd9aasj93j2sdakjas83a@github.com/johndoe/kibana.git/': The requested URL returned error: 403

To handle this situation more gracefully the application should initially verify that the validity and scopes of the access token.
If the access token is invalid it should exit with a helpful error message.

fetch(`https://api.github.com/${owner}/${repoName}?access_token=${accessToken}`, {
  method: 'HEAD'
})