Allow filtering anonymous user in portal -> user management

**Problem**
The user management list could use a filter that does not show anonymouser.

**Appetite**
1 week?

**Solution**
Add a filter dropdown/menu, tick/untick anonymous user

**Rabbit holes**
Backend admin API probably need to support filtering if there is no such feature right now.

**No-goes**
Searching users.

Start at the portal user-management list and inspect the backend admin API to determine how anonymous users are represented and whether filtering is supported. Done means a filter menu can include or exclude anonymous users without adding user search, with the list reflecting the selected option.

Written by the indexing model from the issue text.