syncthing: use service files from upstream

Currently only for the user services as NixOS handles the named system
instances slightly differently.

syncthing and syncthing-inotify are done the same way.

There are 4 parts to this:

 1) Copy in the upstream unit files
 2) Make the nixos module use the definition from upstream
 3) Enable restarting of all instances (system and user) on resume
 4) Allow the traffic in the firewall on default ports if wanted

fixes #18973