Add bulk import for date polls

[PhrozenByte]

Describe the goal you'd like to achieve

As a poll owner, I want to be able to do a bulk import for date polls.

Describe possible solutions

See #2339

Additional context

Since implementing a locale-aware date string parser is incredibly hard, I'd recommend you not to try. You better force ISO 8601 dates. If you do so, date string parsing is as simple as calling moment(string, moment.ISO_8601) (don't use moment(string), as it's a wrapper around the discouraged Date.parse() function).

[dartcafe]

It's on the list, but needs additional validation. Thanks for adding the issue.