Add error handling to check for valid user github data

Check for valid user github token, email and username, before rendering the save as template form. If any of the data is missing, display an alert to the user, and take them to the new github token screen.
Would it be better to throw this logic into something called ```.has_valid_github_token?```? That way we can just use that method instead of the compound logical statements here and in the templates_controller (and keep some logic out of the view).