Enable a public explaination for taking down material to be given

[RichardTaylor]

When a request, or individual message (perhaps even an annotation), is taken down it should be possible for administrators to give a public explanation of the reason in the location where the material was previously published.

This is discussed in the comments at #34 and #209

Note: Sometimes a URL has to be changed, or deleted if problematic material occurs in the URL.

There could be a free text section, as well as perhaps some classifications which could aid site administration and creation of statistics eg.

Status of takedown: Resolved vs Under Consideration
Reason for takedown: Spam, Personal Information, Not a request for information, Potential Defamation, Copyright, etc.

[sebbacon]

From an email discussion:

I think these users would benefit by learning that not all requests
are FOI. This new status could be accompanied with additional info or
functionality to prevent them from pestering the authority with
internal review requests, 20 day limit overdue reminders, and general
further whinges etc. when these no longer apply.

Ah, yes, so the point is that even though such requests are
requester_only, Alaveteli still generates reminder emails, suggests
internal reviews, etc. The new state would cause it to stop doing
such things, so it would avoid giving the user the impression
they could expect a reply.

I think the new state would also disable changes of state by the
user, and cause the "described" flag to be permanently set.

[garethrees]

I think this is covered by the prominence_reason attribute on incoming messages and outgoing messages.

There's a more specific issue to add prominence_reason to an entire request if the whole request's visibility is changed #6746, so closing in favour of that.