First external enrol plugin to be converted to new enrol infrastructure.

Purpose:
1/ create new courses - now separate from user enrol
2/ synchronise or just add user enrolments
3/ education of enrol plugin developers

Features:

• new settings.php
• enrolment synchronisation logic optimised for performance and lower memory usage
• protected enrolments and role assignments
• some code and ideas shared with auth/db

TODO:

• test, test, test