Still needs an introduction and there are some questions to be
answered. Probably ought to demonstrate the code in an event handling
function.

Documentation edits

Revert "Fixes #1856: The default CSS class for hint generated by `ActiveField` is changed to `help-block`"

This reverts commit 7ae4695d.

Documentation updates

Added advanced application configs schema to docs, fixed the fact that there's now common/config/main.php