Add body classes for logged-in WPJM-specific roles

This is just a suggestion and feel free to mark as wontfix if it's a non-standard way of doing things, but when looking at https://github.com/Automattic/WP-Job-Manager/pull/1691 I had a thought that a super-easy fix would be to have a body class that says something like `logged-in-employer` if you're logged in as a user with an Employer role, and `logged-in-candidate` if you're logged in with a user with a Candidate role. WordPress already has a `logged-in` body class, so my suggestion isn't much of a stretch, but it still might be a non-standard way of doing things.

Review pull request #1691 and the existing WordPress logged-in body-class behavior first. The change is done when Employer and Candidate users receive the requested role-specific body classes without changing the existing logged-in class.

Written by the indexing model from the issue text.