Replace "automation" feature flag with "campaigns" toggle and remove configuration flags

Created by: ryanslade

Extracted from RFC 157.

Tasks

• The experimentalFeatures: {automation: "enabled"} flag needs to be removed.
• New site-config property: campaigns.enabled: true|false.
• Add "Disable campaigns" quick action in config editor
• Make sure to disable on dotcom

If campaigns.enabled == false:

• Campaigns are not visible in UI
• Every campaigns resolver needs to check for the flag (see allowReadAccess check right now) and return error if campaigns are disabled.