Allow all time-possibilities for date/time-polls

[mylaeus]

Which Polls version are you running? (see apps page)
Polls Vers. 1.1.6 and Nextcloud Vers. 17.0.2

Steps to reproduce

Create a new poll.
Add a new Date-option

Actual behaviour

You can only select time at the full hour or at .30. Other times can't be selected.

Expected behaviour

I should be able to select whatever time I want to.

As the trains for the participants of our meetings arrive at the full hour and they have to walk about 15 minutes to the office, the meetings normaly start at a quarter past the hour. If I don't state a clear time in the poll, the result will not be correct, as people think, they have to take a train an hour earlier.