Initial support for third-party fallback hub such as Superfeedr for feed subscriptions.

Initial support for third-party fallback hub such as Superfeedr for feed subscriptions.
If set up, this hub will be used to subscribe to feeds that don't specify a hub of their own.
Assumes that the fallback hub will, in fact, handle polling and updates for any feed we throw at it!
Authentication may be specified for the fallback hub.

Example:

  $config['feedsub']['fallback_hub'] = 'https://superfeedr.com/hubbub';
  $config['feedsub']['hub_user'] = 'abcd';
  $config['feedsub']['hub_pass'] = 'ckcmdkmckdmkcdk';

Also:
* Fix for WordPress-RSS-via-Superfeedr-Atom; if we have <author> info but no ID from a native ActivityStreams actor, don't freak out in the low-level processing code that checks for identity matches.
* enhanced messages for low-level FeedSub exceptions if they make it to outside display