Use new endpoint to retrieve users

The new endpoint returns user objects (rather than preference objects
containing user objects), and the user objects now contain preferences.
This change requires notifier to be run against edx-platform commit
bbd6f78 or later.
Improve handling of comments service errors

An exception is now raised on any non-200 response (instead of only on a
500), so the notifier will no longer attempt to continue processing
normally despite an error.
Fix minor bug in forums_digest command

An exception was previously raised if the command were invoked without
explicitly specifying the list of user ids.