Make it possible to put the site in maintenance mode
By specifying the UTC date from which the maintenance shoulds start,
developers can put the site in 'maintenance' mode.
Also redirect to /maintenance if we get a CapabilityDisabledError.
Patch by: Sverre Rabbelier
<html>
<head><title>Something went wrong</title></head>
<body>
We ran into a problem while serving your request, please try again.
</body>
</html>