Users and categories should not be auto-completed based on page names (API action=opensearch) but instead based on their own respective query modules (list=allusers, list=allcategories) because the existence of a description page is optional and neither a subset nor superset of the database tables.
- Pages can exist for inexistent objects.
- Objects often don't have an associated page.
718db58f introduced category editing with autocompletion from opensearch (with hardcoded 'Category:' prefix). This should be switched to using list=allcategories instead.
That way the input field will no longer include results for inexistent categories, and no longer omit results for existing categories lacking a description page.