Add API support for fetching dropped files.

The API now supports fetching all dropped files for a given task
via the `'/dropped/get/<task_id>/all'` URI an also supports fetching
individual dropped files via the `'/dropped/get/<task_id>/<path:path>'`
URI.

Due to the way dropped files are stored on disk, I ended up requiring both the task ID and the path to the dropped file when fetching individual dropped files. The task ID is not strictly required, but I found it made for easy validation of the `root` path that contains the dropped file.