Support local storage for uploads

[ndarilek]

I'm considering packaging Firestorm for the Cloudron platform for one-click installation and integration. Cloudron provides apps with a local storage directory in which to keep files. For those of us who don't use AWS or prefer to avoid it, it'd be nice if files could be stored locally.

I assume this would be a fairly easy change to make, perhaps with activation keyed off of the presence of an UPLOADS_DIRECTORY environment variable. I've only spent about 10-20 minutes with the source, so if there is no conceptual reason this should be difficult then I may be willing to take it on.