Patch to allow linking files with companies. So far a company link was
implied through the project, meaning that if a file was to be linked to a
company a project owned by that company would be required to host the file.
With this patch a file either belongs to a project/task (and the company is
implied), to a company (and there's no connection to any project/task) or
it belongs to nobody.
This has nothing to do with the file owner, in case you're wondering.

This patch requires a DB upgrade (included).