Polls: Add duration for events in the poll

[budachst]

Is your feature request related to a problem? Please describe.
When trying to create a poll for scheduling a meeing, often times the date and starting times are not enough if dealing with a number of invitees, whose schedule is very busy. These invitees often, or almost in our case, demand a precise duration for the event in question.

Describe the solution you'd like
Provide a way to additionally set the duration for an event in the poll.

Describe alternatives you've considered
We tried text based polls, but these are not "calendarish" enough and people generally don't read all the text, if the poll is about finding a date for a meeting.

Additional context
Actually, doodle provides this feature and the adption will be increased and we can nudge peaople towards usingf the poll app on our NC instance, rather than doodle or the like.

[dartcafe]

👍 #194, #201, #404, #426

[dartcafe]

Adding duration and All day option to date options