Handle Revoked Channel Access - SGW Changes

Tracker for changes arising from the Handled Revoked Channel Access feature.

Consistent handling of documents that users lose access to regardless of whether it was because they lost access to it as a result of document removal from all of the user's channels or because the user loses access to the channel  is important.

PRD: https://docs.google.com/document/d/14gawyt9lcHYUlCh0APUt-7V6M6Kkk0zoXi7GOfYiMLM/edit?ts=5fc86642#

Changes:

• whatsnew
• sync gateway's channels page http://localhost:5000/sync-gateway/2.8/learn/sync-gateway-channels.html#remove-from-channel
• sg.yaml
• sync-gateway-admin.yaml
• icr-sgreplicate.adoc
• release notes – breaking change