Add endpoint for download

[qwelyt]

You can now download your models using the endpoint /download. You need to specify the parameters m for which module and format for which format you want it in.

Ex: localhost:5000/download?m=box.py&format=stl
Will produce box.stl for download.

Things of note:

• ~~Unsure about the 404-error thrown willy-nilly.~~ Redone. better return codes now
• Don't know if the getting and setting of module_manager values are really necessary.
• Style. Python is not my strong side. My code looks very different from yours.