Allow organisations to be closed.

Nobody has claimed this yet.

Although it is possible to delete organisations, there is no functionality to allow for organisations to be closed. Closed organisations are those that cease to exist, but whose datasets and other metadata are still important.

Sometimes the organisation's datasets are merged with another, sometimes they're just needed for a historical archive, but deleting them (and thereby removing from public view) is not what is required.

I'd suggest we'd need:
- Allow for marking an organisation as closed (or re-opening).
- Prominently show closed organisations as closed (optionally with a link to the new organisation it may have become part of).
- Show warning when adding to, or editing, datasets in closed organisations.
- Enable moving of an organisation's datasets to another organisation wholesale. Moved datasets should contain metadata about where they have come from.
- Ensure that whenever the organisation is shown, the title is rendered with _("(Closed)") appended, or some other way of ensuring that the title clearly shows the organisation no longer exists.

Start by decomposing the requested organisation lifecycle changes into closure and reopening, closed-state display, edit warnings, and wholesale dataset transfer. Review the existing organisation and dataset flows to identify the affected entry points and how metadata is represented. Done means all listed behaviours are specified, implemented, and covered across the relevant organisation and dataset views.

Written by the indexing model from the issue text.