Add a warning and a command to help user to login

scanner: issue a warning when no auth token exists

This should make the problem more obvious to new users.

scanner: add a swh scanner login command

The command will help people to setup their authentication token.

There are more manual step than we would like, but this pave the way to even more automated method.

login: automatically fetch a token through the API

Test Plan

Tox is happy, and the command work as intended.

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Migrated from D8555 (view on Phabricator)