Allow user selection of poll time zone

[andreas1107]

⚠️ This issue respects the following points: ⚠️

• I agree to follow Nextcloud's Code of Conduct.

Describe the goal you'd like to achieve

As a poll owner, I want to be able to manually set the time zone of a poll, because then I am certain which time zone the poll really uses.

Describe possible solutions

Currently, I have to guess which timezone is used. If I use a privacy browser, then it sets to UTC. If I use my work laptop, it sets it to UTC+1 or UTC+2. If I use someone else's laptop, it uses some third time. And I cannot see this when making the poll. I have to login with a laptop/browser that I know has the intended time zone setting to verify what was set.

Additional context

Please as this is implemented, do not automatically change the scheduled times if the time zone is changed by the poll owner.

Thank you for a great product, I use polls all the time ❤️