[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
@alext is right. For unplanned maintenance we expect users to enter any time in the past (until we consider unplanned maintenance as feature itself).