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.

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 project limit.