Document using tokens in campaigns

Created by: mrnugget

This is part of https://github.com/sourcegraph/sourcegraph/projects/7

We need documentation for the new feature:

• Changelog entry
• Remove/adjust the "code host auth" line in https://docs.sourcegraph.com/campaigns/how-tos/site_admin_configuration
• Update this section https://docs.sourcegraph.com/campaigns/explanations/permissions_in_campaigns#code-host-interactions-in-campaigns
• Update this section https://docs.sourcegraph.com/campaigns/explanations/permissions_in_campaigns#permission-levels-for-campaigns
• Create a how-to on how to use different tokens