Display "Under maintenance" warnings in advance of planned downtime

[edgarcosta]

We should have an easy way to enable a banner notifying users of planned downtime, e.g.

The site will be undergoing maintenance for about 5 minutes at <specified time>.

It is even better if the time can be displayed in the user's time zone.

[AndrewVSutherland]

Tagging this future because with our current setup software and schema changes should require no perceivable downtime (the switch takes less than one second). The only reason we should ever need to schedule downtime is if/when we need to reboot grace, but this is very rare, and certainly does not need to happen in the next month or two (grace is currently all caught up on software updates and no reboots are pending)

That doesn't guarantee that the server, database, and/or grace won't go down at any point in the next month, but if they do it will be unplanned.