Enable using subdirectories for imported clips

For the upcoming shoot, I'd like to be able to specify subdirectories within my storage root to copy to. The command-line flag -storageDirs lets me specify the roots, but from the UI, I'd like to enter a custom prefix/subdirectory inside the roots.

Implementation notes:

Since the take data just records the clip name, the UI will still properly show duplicates. However, the user can probably bypass deduping at the filesystem level.