Use versioning for backups instead of custom hard-coded schedule

#1318 nextcloud/bookmarks@0d7ce3d introduced a backup feature, which got enabled by default (sigh; not going over issues with that now. Should be obvious by now as it got mentioned in other issues as well).
#1779 & #1781 asked for a feature to disable these backups. (which is now possible for a single user)

this needs to be a global admin option to disable it for everyone.

https://github.com/nextcloud/bookmarks/issues/1779#issuecomment-1100946407 wasn't implemented [yet, but the issue was closed already]

So, I'm opening this issue for a global admin option^. And…

I've revised this decision now, and have implemented a regular backup, keeping backups of

• every day for the past 7 days
• every Monday for the past 5 weeks
• every first Monday for the past 6 months

https://github.com/nextcloud/bookmarks/issues/1318#issuecomment-1065080553
It's nice to have some sort of backup strategy, but why aren't you using existing versioning in nextcloud (which can be configured by the admin). Quick-Docs reference

Hence I propose updating the same file, instead of creating&deleting yourself. This would allow much better user-controlled backup behavior.

edit.: fixed issue linking;

Start with the backup feature introduced in #1318 and review Nextcloud's file versioning documentation linked in the issue. Determine how the custom schedule can be replaced by updates to the same file and how a global administrator option should control it. Done means custom backup creation and deletion no longer override administrator-configured versioning behavior.