[Feature] Additional host paths for library import

As far as I can tell, Nefarious looks in its own Host Download Path when trying to import my existing media library, which seems like a… kind of weird assumption?

My media library lives on a NAS that the torrent box doesn't have write access to (and I intend to keep it that way). I haven't tested this, but for now I intend to try symlinking the network mount into the Nefarious download path. I have no reason to think that won't work, but I'd call it a workaround.

Personally, I would expect there to be a config option in the .env file for path to media library, which I assume would then be mounted read-only in the container. I don't know if it's possible to add paths from inside the already-running container, but having having an "Add library path" option right near the "Import TV shows" and "Import movies" buttons would be handy if it's feasible.

Ideally, it would be nice to be able to specify an arbitrary number of import paths. Personally, my media library is optimized for Kodi, so it's split into separate paths for each family member's TV shows, and then again for each family member's movies, etc. I don't know whether it would confuse Nefarious to add the parent directory, which also includes music, random video content, music videos, and so forth. I expect I'll find out soon, though. ☺

Start by tracing the existing “Import TV shows” and “Import movies” actions, then inspect the .env and container path configuration described in the issue. Done should support multiple configurable library paths, including read-only NAS mounts, and expose an “Add library path” option if the existing interface supports it.