Add support for a HOMEPAGE_INTERSTITIAL env variable to allow us to put up a status page

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I've updated our dev db to Postgres 16 yesterday and it takes the db offline for about 8 minutes.

I was planning to do the same next Sunday morning but rather than have the server show an obscure error page we could put up a simple, configurable interstitial on the home page via an env variable HOMEPAGE_INTERSTITIAL. Its content can/should be Markdown.

This could also be useful in case we ever hit db issues, because this page can be served when the db is down.

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That'd be great, it's a bit tight the way it is 👌