Improve Poll Page Preview

[lukasrad02]

Currently, the preview of a poll cannot be used to preview a poll including its options:

• When viewing the preview before or after the voting period, one can only see the poll description and the notice You've accessed this page outside of the voting period., but not the poll options. I would wish to be able to see the options in the preview for easier reviews.
• When viewing the preview during the voting period, rendering fails with the error 'FakeQuerySet' object has no attribute 'intersection' at {% if page|can_vote:request.user %}. I would wish for the preview to ignore the permissions of the user and always show the options.

During these changes, we should forbid submitting actual votes if this is not already implemented (couldn't check due to the error mentioned above).