FIX Load _config.php's after static config manifest

This allows more sophisticated handling of config alterations
in _config.php. One example is additions to DataObject::$db
based on configuration which requires some processing.

See https://github.com/unclecheese/TranslatableDataObject/blob/master/TranslatableDataObject.php
FIX Incorrect parsing of T_STRING values in class spec parsing

* Due to missing break, the T_STRING case would fall through to array.
* The values were being added to the wrong variable.
* Added missing support for missing null values.
FIX ConfigStaticManifest persisting access level after parsing a static
FIX Allow FALSE in Config API, call remove() will NULL key on update()
FIX Clean cache on Config->remove()
Formatting and docs changes to configuration.md
Disable RequestHandler->handleAction() deprecation until we have fixed LeftAndMain usage
Allow for short versions in Deprecation::notification_version()
API Marked statics private, use Config API instead (#8317)

See "Static configuration properties are now immutable, you must use Config API." in the 3.1 change log for details.
MERGE More config changes