Expose projects_limit through users API if UserFull.

If the authenticated user of the API is an admin, they will be using the Entities::UserFull set of exposed params. This patch exposes User.projects_limit and updates the relevant documentation and tests.

As background justification for this proposed change, I had a use case to want to see how many projects a user is limited to for our site's GitLab installation. We do space auditing as a multiplier of the projects limit.
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