syncthing: expand declarative configuration

This expands the Syncthing configuration to allow declarative
settings. Code mostly pulled from the Nixpkgs module.

Changes compared to the NixOS module are:

Removed the following options:

- user, group, systemService: Unnecessary since Syncthing always runs
  as the user declaring the configuration.

- dataDir configDir, databaseDir: Pointed to ~/.local/state/syncthing,
  the default Syncthing directory.

- openDefaultPorts: We don't have access to the system firewall.

Furthermore, multiple changes to systemd services were made to
maintain consistency with other Home Manager modules, sandboxing
options might need to be reviewed further.

Fixes #4049

Author: karaolidis