Issue #1890784 by alexpott, YesCT, heyrocker, tim.plunkett, EllaTheHarpy, beejeebus: Refactor configuration import and sync functions.

Issue #1947814 by vijaycs85, Gábor Hojtsy, tim.plunkett: Fixed New config entities often save as langcode: und incorrectly.

Issue #1515312 by heyrocker, beejeebus, alexpott, xjm, realityloop, sun: Added snapshots of last loaded config for tracking whether config has changed.

Issue #1831774 by damiankloip, tim.plunkett, Gábor Hojtsy, alexpott: Fixed Config import assumes that 'config_prefix' contains one dot only.

Issue #1826602 by damiankloip, xjm, sun, dawehner, EclipseGc: Allow all configuration entities to be enabled/disabled.

Issue #1697256 by heyrocker, alexpott, sun, xjm, webchick: Create a UI for importing new configuration.

Issue #1788462 by samhassell: Remove references to 'active store' in comments for configuration system.

Issue #1741804 by beejeebus, sun, chx, alexpott, matason: Implement a config import/staging directory.

Issue #1668820 by sun, xjm, tim.plunkett, fgm: Concept, base class, and interface for configurable objects.

Issue #1711070 by kid_icarus, tim.plunkett, xjm, droplet, zendoodles and cam8001: Convert tests to use ::$modules property instead of parent::setUp($modules).

Issue #1447686 by sun, beejeebus, Pol, heyrocker, David Strauss: Allow importing and synchronizing configuration when files are updated.