Replace by google blockfactory

Replace the actual toolbox/workspace editor (https://github.com/UCL-INGI/INGInious-problems-blockly/tree/master/inginious-problems-blockly/static/blocklyfactory) by blockly's own blockfactory ().

The goal is that the stubs of blockfactory be editable in multiple languages so that the stub codes are persisted in a specific file (TBD). It's not yet clear if we want it to be a js file or a yaml-ish file from which the js file would be generated on the fly.