[TAV-313] Issues with singleton output ports.

Certain rare cases of particular data within a singleton output port were
causing errors.
Implement basic notification/interaction machinery.

Get notifications from Taverna Server and present the linked interactions in an
iframe within the individual run page. Interactions are posted one at a time,
even if they are read from the feed at the same time, to avoid hitting this
bug/behaviour: https://jira.biovel.eu/browse/TAV-311