A model to represent transaction downtimes

www.agileplannerapp.com/boards/173808/cards/8805
Artefact can have downtime

www.agileplannerapp.com/boards/173808/cards/8805

downtimes reflect service availability and can be
of multiple types: scheduled, ad hoc, or repeating.

hence, it seems appropriate that downtime belongs
to the `Artefact`. it ensures that re-editioning an
artefact doesn't affect downtime.