add mongoengine date field

[wangsha]

fixes: #1921

[samuelhwilliams]

This feels like a useful change in principle, but right now it doesn't work.

The filter value popup shows a datetime widget, and should probably only show a date widget.

Ideally there should also be some good test coverage for the new field to provide some basic confidence that it works correctly.

I am willing to support this addition in principle, but it is not ready in this current state.

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[samuelhwilliams]

I'm going to close this for now, as it doesn't work and is a little old now. There's also some questions around how supportable/maintainable the flask-mongoengine integration is, as it's not really supported any more and that will start to cause issue soon.

Feel free to open again if you want to raise this again.