Design: Set up defaults for Apple::Config publish_enabled and publish_blocks_rss and add UI facility.

[svevang]

Subsequent to the Subscriptions UI creation flow, the user should be in a state that's suitable to upload audio. But currently, the Apple::Config defaults to publish_enabled == false and sync_blocks_rss == false.

Publishing needs to be enabled on the config in order to upload audio. Also, the 'sync blocks RSS' setting should be in a more conservative stance, meaning that if someone is setting up subscriptions, they probably expect that the subscription only audio is uploaded first -- in the case of Delegated Delivery blocking the release of the non-subscriber audio.

• switch default to publish_enabled == true
• switch default to sync_blocks_rss == true
• optionally give the user an affordance to be able to toggle these settings (with appropriate caveats and warnings).