don't drop into browser flow when no auth detected #1511

Currently when gh detects that no host is configured when trying to do an API call it automatically puts the user in a browser-based authentication flow with the goal of returning them to what they were doing upon success.

While it's got us this far and can often feel smooth, it's been a pain point for a variety of users. This issue covers switching to a more explicit approach where, for commands that require authentication, we stop and print helpful instructions for the ways a user can authenticate.

The flow should look something like:

$ gh pr status

Hi! gh is not yet authenticated with any GitHub hosts.

For interactive authentication, please run `gh auth login`. That command will allow you to either log in using
a web browser or paste an existing token.

If you already have a token to use and don't want to store it anywhere, you can set the GITHUB_TOKEN
environment variable.

cc @ampinsk for wording/formatting suggestions.