fixes #3892 - process REMOTE_USER_GROUP_N and REMOTE_USER_GROUP_#, add user to groups based on external user groups.

fixes #4462 - extending the /users/login handling to process REMOTE_USER through intercept

fixes #3528 - Populate first name, last name, and email, if available.

Fixes #3753 - always use main_app to determine paths for running an isolated namespace plugin

fixes #3312 - support /users/extlogin as a login page based on REMOTE_USER

Fixes #3043 - Support Signo on API level using BasicAuth

fixes #2988 - merge authentication code, enables REMOTE_USER auth on new API controllers

fixes #2411 - move files in /models to /concerns, /services, /mailers, /observers