Add app config for default template path

[tcitworld]

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• [ ] Tests (unit, integration, api and/or acceptance) are included
• [ ] Screenshots before/after for front-end changes
• [ ] Documentation (manuals or wiki) has been updated or is not required
• [ ] Backports requested where applicable (ex: critical bugfixes)

[tcitworld]

I used an app config instead of a system config value as this can be sensitive (could be a bad idea to use a path mounted on external storage for instance) and I don't think we document those. But otherwise I don't have any specific opinion on this.

[juliusknorr]

App config seems fine, but documentation would still be good. Maybe at https://docs.nextcloud.com/server/latest/admin_manual/configuration_files/default_files_configuration.html

[PVince81]

/rebase

[tcitworld]

Docs are in https://github.com/nextcloud/documentation/pull/9569

[juliusknorr]

/rebase

[juliusknorr]

Failures seem unrelated.