licensing: add new plans and enforce SSO

Created by: unknwon

This PR does two things:

1. Adds new plans "business-0" and "enterprise-1".
2. Requires a valid license for enabling any non-builtin authentication provider, i.e. taking SSO away from Free instances.

Warning logs are printed to the console and site alerts are shown to the site admin:

Test plan

Unit tests and manual e2e tests.

Part of https://github.com/sourcegraph/sourcegraph/issues/40064