Authenticate menu option should disappear after successfully authenticating with GitHub

[meaghanlewis]

Description

The GitHub menu always shows the Authenticate option.

Steps to Reproduce

1. Go to GitHub > Authenticate
2. Sign in with GitHub
3. Go to GitHub > Authenticate again

Expected behavior:
The Authenticate option should not be shown in the GitHub menu.

[shana]

So this menu item is kinda temporary for debugging purposes while we're in alpha, to simplify access to the UI. It's not really meant to be around for the release version - the "real" menu lives in the GitHub toolbar and changes depending on your logged in status.

Maybe we will need a menu option for credentials at some point, but for now just ignore this one beyond the basic "does it open the authentication window?" functionality.