Implement new filedrop upload

We want to sunset our existing filedrop integration in favor of the new pretalx "private upload" feature on cfp.cccv.de.

This requires a webdav endpoint (https://github.com/drakkan/sftpgo might be an easy option) and then we have to

1. Watch changes in the directory and update our data model accordingly OR
2. Remove the in-database list of files and just list the files directly from the FS

Currently, we just symlink the filedrop cache for our "second language conference". We must be able to do have something analogous to that.