[Extensibility] MAGETWO-36838 and Unit Test Coverage

Conflicts:
	app/code/Magento/Config/Block/System/Config/Form/Fieldset/Factory.php
	app/code/Magento/Config/Test/Unit/Block/System/Config/Form/Fieldset/Modules/DisableOutputTest.php

[Vanilla]   (MAGETWO-38867) Improved declaration of widgets with `mixins` node

Merged with failed (red) L1/REST EE build and  failed (red) FAT/CreateGroupedProductEntityTest build. Reffering to those fails are know isssue and also fails in current mainline.

[Ogres] Bug-fixes

Merge remote-tracking branch 'mainline/develop' into PR_Branch

Conflicts:
	dev/tests/static/testsuite/Magento/Test/Legacy/_files/obsolete_classes.php

[Extensibility] Sprint 53 Stories and Bugs

[WD] Update Content in Magento2 + Bugs

- Merge remote-tracking branch 'origin/develop' into PR

- Conflicts:
	- app/code/Magento/CatalogRule/Controller/Adminhtml/Promo/Catalog.php
	- app/code/Magento/GiftMessage/view/frontend/web/template/item-level-gift-message.html
	- app/code/Magento/GiftMessage/view/frontend/web/template/order-level-gift-message.html
	- app/code/Magento/Reports/Block/Adminhtml/Filter/Form.php
	- dev/tests/functional/tests/app/Magento/Customer/Test/TestCase/CreateExistingCustomerFrontendEntity.php
	- dev/tests/functional/tests/app/Magento/Install/Test/Constraint/AssertSuccessInstall.php

[SOUTH] Shopping cart + Bugs

Conflicts:
	app/code/Magento/Checkout/composer.json
	app/code/Magento/Downloadable/composer.json

[Troll] Concurrency enhancements & bugfix

Add gitter.im badge to ReadMe.

- CR changes

[Nord] Sprint 22

- fixed randomly failed integration test

Merge remote-tracking branch 'origin/MAGETWO-38833-fix-error-messages-theme-uninstall' into PR_Branch

Conflicts:
	composer.lock

Merge remote-tracking branch 'origin/MAGETWO-38994-Eliminate-code-duplication-ComposerInformation' into PR_Branch