Add atuin service

[Jacques-z]

https://docs.atuin.sh/self-hosting/server-setup/

I didn't forget #157, and I will try to handle both soon.

[Jacques-z]

@moan0s Thanks for the instructions in the other issue! This enabled me to create an atuin one by reading its doc and comparing gitea and gotosocial.
Unfortunately dunno how to test, so could you please help with that?

Here're something concerning:

• I don't know if the modification in ExecStartPre --mount is correct, especially the config entry. I wrote because I saw something like volumes: \ - "./config:/config" in the doc
• timeout_start_sec was set to 0.
• ATUIN_HOST: "0.0.0.0" became ATUIN_HOST={{ atuin_hostname }} and atuin_hostname: ''
• line 28 and 48 in install.yml, dunno how to determine which one to use.