Use non-templated systemd service

The previous setup was broken

This commit also adds some code to check that the database connection is
valid before it starts, as well as refactors the code that splits
between starting and external or internal server.

src/core/common.rs

@@ -1,5 +1,6 @@
 use anyhow::Context;
 use nix::unistd::{Group as LibcGroup, User as LibcUser};
+use std::{fs, os::unix::fs::PermissionsExt};
 
 #[cfg(not(target_os = "macos"))]
 use std::ffi::CString;