[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
Do we require `start_time` to be in the future.  I can imagine a use-case where we need to add a notice for a service that's currently down.

Also, these validations should probably only be applied on create.  We want to avoid the case where an entry loaded from the database can be invalid (which could cause problems for saving the `Artefact` etc.