Clarify that you have to log in to Github to verify your identity

It's sort of a stupid error, but a mention that to publish your Gist you have to log in to Github would have saved me a few minutes. I was not logged in in the window that popped up during the verification process and did not notice that I wasn't.

Start with the user-facing message shown during the verification process when publishing a Gist. Update the wording to explain that the user must be logged in to GitHub, then verify that the message is clear during the publish flow.

Written by the indexing model from the issue text.