Add atuin server support

Adds the published atuin-server.service systemd unit to the package with supporting files to allow for self-hosting of the sync server.

The following files are also added:

• /etc/atuin/server.toml: Default server configuration file.
• /usr/lib/sysusers.d/atuin-server.conf: Create unprivileged atuin system user and group. Patched from upstream to support fully locked users introduced in systemd 257.
• /usr/lib/tmpfiles.d/atuin-server.conf: Change the mode and ownership of the config file to 0640 and atuin:atuin respectively. The config file should not be world-readable as it could contain database credentials.

Additionally, postgresql has been added to optdepends. I also added the --parents flag to the mkdir call in prepare() as I noticed that would cause the build to fail if the directory already existed during testing.