Provide an indication that staff are viewing a restricted item

[gestchild]

Relates to #5761

Once staff are able to view restricted items it would be useful to provide an indication on the page when they are doing this.

In the past there has been confusion when staff are able to see things that members of the public are not and we should make it more obvious why this is the case.

This will be need on the items page, e.g. https://wellcomecollection.org/works/pzmyhgsk/items

and possibly the work page https://wellcomecollection.org/works/pzmyhgsk/

Normally, the work page won't show a link for restricted items, but will do for staff who are logged in