[Discuss] Add downtime to artefacts

**we'll merge this PR once all changes related to this feature are in, so that we can release them in one go.** 

www.agileplannerapp.com/boards/173808/cards/8805

As a content designer
I want a better way to indicate on transaction start pages when a service is down for planned maintenance
So that I don't have to re-edition multiple pages and push these through workflow, then repeat the process again once the downtime is over
I'm unsure about embedding this into the artefact.  The artefact model it likely to go away as we pull publisher and panopticon apart.  It'll be easier to handle when that happens if these notices are stored in a separate collection.