Add button to close user account

[garethrees]

We now have User#closed_at that prevents users signing in.

We should have a button that allows admins to permanently close a user account.

Required by #3148.

[RichardTaylor]

It should be made clear what this "closed" status means. (Prevention of login, and alert mails, though not a prevention of transactional emails / notifications relating to a users' own requests?)

Perhaps notifications relating to a users' own requests should also be prevented?

A user who can't login to classify a response shouldn't be asked to do so. (Comment arises from support mail on WhatDoTheyKnow from a user in this position)

[mdeuk]

A user who can't login to classify a response shouldn't be asked to do so. (Comment arises from support mail on WhatDoTheyKnow from a user in this position)

I've raised #5090 for this as we've had another WDTK user complain about being asked to do just that this weekend.

[garethrees]

It should be made clear what this "closed" status means.

Yes. We already have a "Danger Zone" on a admin user edit page, so we can make it more like the GitHub approach:

We'll want to be able to unfold details for each dangerous action:

[RichardTaylor]

I think this was done by #4869

[garethrees]

I think this was done by #4869

That was Close and Anonymise. This issue is just for a "Close" button (closing does different things to adding ban_text).

[garethrees]

Closing in favour of #7646.