Fetch additional license information via API

Problem to solve

Currently, licensing information for a GitLab instance is accessible through the /license API endpoint. See the docs for more details.

This endpoint is missing important licensing information that we should add. This is intended to help admins manage and check instance activity via API, especially for admins who are managing multiple instances.

Target audience

GitLab administrators.

Proposal

Add the following to the response returned when we GET /license:

• License plan (e.g. Starter, Premium, Ultimate)
• Maximum users
• Users over license

Links / references