Log changes in configuration settings

Original description: Log could record all admin activities.

Trigger an event from inside set_config() function. It will cover the most of the admin actions but still not all. Follow the rest in Epic MDL-45773
In this issue make sure that changes in "Frontpage settings" are also logged - they are stored in 'course' table and not in 'config'

Note that there is an existing function add_to_config_log() that inserts a record in table config_log on SOME changes of configuration settings. The contents of the table is shown in report_configlog. To avoid duplication of data in log tables I would recommend to deprecate this function and adjust report_configlog. This is open for discussion.
When making decision take into account MDL-40224 - users want to see both changes to administrative settings AND role assignments in one report. It can be possible either if we record both in config_log or retrieve both from standard log. Also see MDL-45888, filter_set_global_state() inserts records in config_log as well