Show used placeholders and placeholder limit in Usage Quotas section

About

The number of placeholder users available can affect any further imports into a namespace.

We would like to let people check their current placeholder user count and limit.

Proposal

We would like to add this information to the Usage Quotas of projects and groups #486691 (comment 2099048316).

We would only show this information if there are placeholder user limits applying (we can use #unlimited?).

It should display on groups Usage Quotas sections.

The backend can provide the placeholder user limit, and how many placeholders they have used.

The frontend of the Direct Transfer page will be updated with an info alert #486691 (comment 2112279590) to let people know to check Usage Quotas.

We would add documentation of this to the user mapping docs.