Add vendor prefix to contao dependency.
Merge pull request #802 from discordier/patch-1

Add vendor prefix to contao dependency.
Added a branch alias for the 2.0 dev release
Added missing method to product interface
Fixed wrong reference on user object
Fixed relying on the column "id" to exist
Only include variants if there are any
Fixed row class in checkout step address (#791)
Fixed missing payment templates for PSP payment methods
Cleaned up import statement
Fixed variant generation did not work
Do not try to create attributes if there is no type is set
Database was not updated (fixes #800)
Updated changelog
Only replace contao-legacy/isotope in self.version.
Allow contao/core since 3.2-RC1 in requirements.
Fixed the drag and drop feature in mediamanager uploader
Fixed warning when creating product without having a product type (#804)
Fixed namespace issue in BasePrice model (#808)
Fixed namespace issue with Environment class