Improve app support -> Implement YNH level 4 (up to 7) guidelines

Global issue to track what needs to be done to improve this app, and make it a fully fledged standard Yunohost app. Forum topic: https://forum.yunohost.org/t/codimd-hackmd-fork-official-topic/11110

To be done (in an approximate order of priority, but any contribution is warmly welcome !):

[x] Install and removal (level 2): #11 and #19 fixed this, thanks to @alexAubin and @Lab-8916100448256 ! :tada:. Still some tests to be done on various configurations…
[x] Improve install robustness, we should at least look into --line-match for systemd_action to check if service started, otherwise test on the CI may fail because of 502 errors
[x] Upgrade working (level 3) #22
[x] Backup&Restore working (level 4) #21
[x] Set app status to "working" in YNH community.json https://github.com/YunoHost/apps/pull/914
[x] Handle non domain root install #24
[x] Level 5 specs
[x] In Yunohost-Apps repository (Level 6)
[x] Level 7 specs

YunoHost-Apps / codimd_ynh