feat!: Make the url signing secret a mandatory config option

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This is required for allowing the web office to download images to
insert into documents.

The secret is generated by `opencloud init` and the server refuses
to start now without a secret being set. (Breaking Change)

Also the setting is now moved to the shared options as all involved
services need the same secret to work properly.

Related: https://github.com/opencloud-eu/web/issues/704

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