file names in blockly tasks

In the subproblem editor, there shoudn't be sections to insert files. The files should have fixed names and be edited in the "Task Files" tab. The names of the files should be suffixed with the subproblem id so that we can have more than one subproblem in one task :

- "Files (Interpreter, Visual execution)" : There should be only one file named by example visual-SUBPROBLEM_ID.js containing the interpreter code + the animation specific code. The name of this file should be fixed and documented.

and

- "Additional blocks files" : This file should also have a fixed name or depending on the implementation choice in https://github.com/UCL-INGI/INGInious-problems-blockly/issues/15 a file containing only the stubs generated by the blockfactory to be added in the workspace later.