support for notifying users of current or scheduled downtime

[turadg]

What is the Problem Being Solved?

Notification banners have to be deployed manually. Some dapps aren't deployed by OpCo and may not get the banner in time.

Description of the Design

Provide a utility for any dapp to notify its users of downtime (either current or scheduled).

For example, some URL provides a JSON with a description of notices. ui-kit provides a utility to query and render notices.

Probably the way Agoric/dapp-psm#94 does it.

Security Considerations

Scaling Considerations

Test Plan

[otoole-brendan]

Latest suggestions include adding a banner to dapp-inter-jumper which wouldn't require an EC vote - we could deploy it ourselves at short notice which has a massive benefit.
We could add banner context to https://main.agoric.net/network-config we’d be in control of the banner too.
'Yes we can put notice text in network-config and have a widget to display it in dapps.'

'Under normal circumstances the jumper redirects to the ipfs-hosted app very quickly, we might want to make the user click a "proceed" button or something if there's a notice we want them to see'