make status.s.o status discoverable from archive.s.o

We have started using status.s.o, and that's great for documenting outages. Currently it is not easily discoverable from archive.s.o users though. As a concrete example: during the current DB outage we have received "bug reports" about save code now not working at info@s.o, which could've been avoided if status.s.o were more easily discoverable.

Concrete proposal: when the status of some service [that might impact archive.s.o, although that is probably "all of them" right now] is not green on status.s.o, we should add a tiny "error" banner on top of all archive.s.o pages, with a brief error message and linking to status.s.o for more information.

Probably the best way to implement this is with a client-side javascript that asynchronously polls status.s.o (assuming that passes muster with the JS security model…), so that it is not hindered by caching mechanisms and don't make rendering archive.s.o dependent on status.s.o.

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Migrated from T2402 (view on Phabricator)