Auto-cohort plugin

Automatically adds users into auto-created cohorts, with names that derived from users’ profile fields.
This local plugin automatically adds users into cohorts, with names that
derived from the users’ profile fields.
Auto-created cohorts are made in the top-level system context (CONTEXT_SYSTEM).
This plugin is derived from, and intended to deprecate, the earlier auth_mcae
plugin by Andrew Kama.