feat(storage): add size of appdata folder to statistics

[tcitworld]

App data is used for a lot of things and the preview folder especially can grow very much. This helps identifying where storage growth comes from.

Also refactor to use IAppConfig in related files.

[nickvergessen]

Please try the maintainers: https://github.com/nextcloud/serverinfo/blob/master/.github/CODEOWNERS#L1

[tcitworld]

@JuliaKirschenheuter @kesselb why are you code owners only on the /appinfo/info.xml file and not on everything? I didn't have the reflex to check that.

[nickvergessen]

why are you code owners only on the /appinfo/info.xml file

To keep the notifications manageable, but they are still supposed to respond to issues and PRs regularly.

[come-nc]

filecount would be useful as well, no?
On my personal instance the issue with previews was the number of files cluttering oc_filecache because of leftover previews.

[tcitworld]

I can add this, yes.

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