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.

Start by reviewing the subproblem editor and the "Task Files" tab, then read issue #15 for the implementation dependency. Done means removing file-insertion sections, using fixed names suffixed with the subproblem ID for the visual and additional block files, and documenting those names.

Scritto dal modello di indicizzazione a partire dal testo della issue.