Implement DI container
Merge branch 'develop' of https://github.com/getgrav/grav into feature/DI

Conflicts:
	composer.json
	index.php
	system/src/Grav/Common/Grav.php
	system/src/Grav/Common/Themes.php
	system/src/Grav/Common/Twig.php
Add missing vendor dependencies
Add DEV/PROD parameter to debugger constructor
Convert DI variables to lower case
Fix exception using wrong namespace in Assets
Use similar way to get Grav context across the classes
Merge branch 'feature/DI' into develop