Accessing and receiving files via FTP

From comment in #270

- Idea is to create a FTP server that connects to docspells http endpoints exposing the files. Items can be represented as directories; must be read-only
- The other way would be possible to: have a dedicated directory that allows to "ftp put" files into, which would be imported into docspell

There is Apache Mina for the hard parts already providing a FTP/SSH java library. This would be a separate project and not integrated into docspell itself.