Barcode generators

[thomasjthomasj]

Provides a framework for generating barcodes for units. Currently only supports Code 39 and EAN 13 but left open to add more in the future.

This PR includes:

• Product\Barcode\CodeGenerator namespace (I wanted to call it Generator but there's already a class called Generate 😤)
• GeneratorInterface interface representing a barcode generator
• GeneratorCollection for storing barcode generators
• AbstractGenerator for basic generator functionality
• Code39Generator for generating CODE 39 barcodes (but really it just returns the unit ID)
• Ean13Generator for generating EAN 13 barcodes
• Exception\BarcodeGenerationException to be thrown if a barcode cannot be generated, and caught to relay that information to the user
• Product\Barcode\ValidTypes class of statics for checking that barcode types are valid
• Product\Unit\BarcodeEdit class for generating a barcode and saving it to the database
• Adds event listener after unit has been saved to generate and save a barcode
• Allows null on barcode column in product_unit column again
• Show flash message on unit creation
• Adds barcode-generator option to barcode.yml fixture
• Unit tests for barcode generators, with 'barcode' testsuite
• Update Cog requirement to 4.10

[kuiche]

Could Product\Unit\BarcodeEdit have a second method to allow custom barcodes? Currently the barcode edit controller uses a Unit\Edit object to save the custom barcode which does a lot of unnecessary stuff. It would be good if this method could validate the given barcode somehow too.