Make clearer to authorities where requests are embargoed

[WilliamWDTK]

On on-site help pages, and explicitly stating so in the footer of emails from requests concerned.

[RichardTaylor]

I think there was an active decision not to do this, to deter additional scrutiny of such requests, or for them to be otherwise treated differently.

It's worth reconsidering though.

[mdeuk]

I ponder if a mechanism that might work would be a 'check this email address is valid' tool - e.g. provide the email address and Alaveteli will confirm if the request is legitimate or not.

This would follow our existing advice, in that we never directly confirm a request is embargoed - but we will confirm if the address is valid (and advise that "not all requests are published").

Longer term, it would be good if we were able to implement a variant of #7351 (perhaps via a "verified" account that could also be used for https://github.com/mysociety/alaveteli/issues/6791), as this would allow FoI officers to self-serve to a greater extent, reducing a bit of friction for them, and a bit of admin time for us.

#3888