Show a hint to the user if sharing is disabled

[juliusknorr]

If sharing is disabled for apps on the Nextcloud server, the user should see a message that adding users to the board is not possible then. (ref #644)

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[jakobroehrl]

@juliushaertl: How can I query the server to find out if sharing is disabled?

[juliusknorr]

@jakobroehrl You should be able to use oc_capabilities.files_sharing.api_enabled in the frontend to check if sharing is enabled. Note this will require us to increase the Nextcloud minimum version required for deck, since it is only available on NC14+.