Authentication does not expire when the GitHub user signs out.

[odisseus]

Steps to reproduce:

1. Sign into GitHub as user A.
2. Go to a site which uses ghcomments and add a comment.
3. Sign out of GitHub and sign in as user B.
4. Refresh the site and add another comment.
   The comment will be added on behalf of user A instead of user B.

Also there seems to be no way of signing out of the ghcomments app, other than revoking its access rights.

[odisseus]

A possible solution is to store the authentication token in sessionStorage rather than localStorage.

[odisseus]

bump

[wireddown]

I'm confused about steps (1) and (2): how does logging into GitHub also log you into a GPGC site?

It seems the defect here is your aside: "there seems to be no way of signing out of the ghcomments app."