Think about adding an option to permanently delete files

[jdbocarsly]

Currently, there is no way to actually delete a file. When the "x" is pressed next to a file in the webapp, the reference to the file is removed from that sample, but the file itself remains on the server, and the file entry remains in the db.

We should think about whether this is the desired behavior, or in what circumstances we want to actually fully delete files.

This is complicated because of the possibility that a file will be used in multiple samples.

[ml-evs]

We should probably have a search index over the files database anyway, so files that were not uploaded as part of that sample can then be attached to other samples. If we store a hash of each file too, we can then quickly check for duplicate files and prevent uploading the same file multiple times.