Change systemd funkwhale.target to service

Currently there is a funkwhale.target file that starts the three services funkwhale-server, funkwhale-beat and funkwhale-worker.

I don't know what was the rationale for having a .target instead of a .service, but targets cannot be enabled, for funkwhale to be started automatically at startup.

I am the maintainer of the funkwhale Arch (AUR) package and I modified the 4 systemd files to make it work with a funkwhale.service file, see the files here: https://aur.archlinux.org/cgit/aur.git/tree/?h=funkwhale-venv