Add 'warning header' feature #205

This can be done post-launch - it would be nice to have a banner at the top of the website for machine outage warnings. The warning period should only last a week or so.

Edit from Neil: The approach I think we should take for showing the banner is to specify a start and end date/time, and show the banner based on that. This way, we could also keep historical banners, and pre-schedule banners as well.

Additional notes from our meeting on Jan. 19/2022:

The warnings can be stored in .md files in a subfolder under the content folder, similar to how news and events are stored.
For now, emit an error at build time if any warnings have an overlap in the time period when they're supposed to be displayed.
The warning banner should appear at the top of all pages (similar to how the Navbar appears at the top of all pages).
For now, we do not need to display an X to close the banner or anything.
We have a form-invalid colour defined in pages/_app.css that currently is not being used anywhere - perhaps this colour could be modified slightly, renamed, and then used as the background colour for the warning banner.
We will not submit a formal design request for this feature, however please send a screenshot in the ~website-design channel on Mattermost once the frontend has been implemented, in order to get feedback from the Design team.