SentManager: rework handling to allow for user specified source

SentManager: rework handling to allow for user specified source

* The handling of SentManager now allows for a parameter in config.yaml
  called sent_source.  This parameter should be the URI of the source
  that the users wishes to store outbound mail in.

* The default is sup://sent

* A FatalSourceError is raised in the event that sent_source is
  set to a source type that doesn't support the store_message() method.

* SentManager and SentLoader have been somewhat decoupled.

Signed-off-by: Ben Walton <bwalton@artsci.utoronto.ca>