Rename "hidden" polls to "private" polls, "public" to "open"

[dartcafe]

Replace the term hidden poll with the term private poll, because it better corresponds to the term public poll as an opposite and it describes more precise, that the owner posseses the access control.

While thinking about it, the term public poll is also a little bit misleading, because the poll is not set to public but to visible to all users, regarding the group settings. So we will also rename "public poll" to "open poll", indicating, that this is visible to other but not necessarily in public in the meaning of accessible to users outside of the site.

Any proposal for better terms is appreciated.

[github-actions]

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