Documents last edit/update

Implements #318 (closed)

Currently a document is considered as updated when:

• Metadata is changed (description, name, category, doc type)
• File added/removed
• File updated via API endpoint

Perhaps it makes sense to only mark as updated when Files are added/removed/updated or description is changed. The other changes aren't really relevant

Also changes front-end to match the style of exam answers:

For new docs where the creation date is known, hovering over the updated label will show the creation date: