wireguard: add each peer in a separate service

Before, changing any peers caused the entire WireGuard interface to
be torn down and rebuilt. By configuring each peer in a separate
service we're able to only restart the affected peers.

Adding each peer individually also means individual peer
configurations can fail, but the overall interface and all other peers
will still be added.

A WireGuard peer's internal identifier is its public key. This means
it is the only reliable identifier to use for the systemd service.