[Feature] Auto delete old polls

[CarliJoy]

In my opinions polls are per definition short lived. There should be no reason to keep them longer as needed to establish an result.

To increase privacy, the user should have the option to limit the lifetime (add an auto deletion date, no matter if the user is registered or not) of a self created poll.

The administrator of a page should be able to set a maximal lifetime of a poll (or deactivate it).

I will create a PR in the upcoming weeks.