FE - User Contribution Mapping - filter for placeholder users in the admin view

Problem

Currently, in the Admin view for instance members, placeholder users are shown under the Users tab and don't have their own Placeholders tab.

This was noticed in our test session and just recently was reported as confusing behavior from a customer.

Update: It has been decided to only add a filter that shows the list of placeholders. See this comment

Proposal (Updated)

Add filter in the UI so that Admin can filter for type=placeholder users in the filter bar.

Add filter in the UI so that Admin can filter placeholders out. By default the filter should be on, meaning by default the placeholders are filtered out.