IN PROGRESS - issue CORE-4126: Add system messages display area to DE.
https://pods.iplantcollaborative.org/jira/browse/CORE-4126

The messages DTO now contains information on whether or not the user as
already seen the message.
The system message cache fires a messages updated event when new
messages arrive.
Adapted the DE presenter to use the IplantAnnouncer to display new
system messages messages.