(core) Convert datetime objects in Date columns to timestamps at midnight of the date

Summary: Changes Date.do_convert to fix a bug noticed in https://grist.slack.com/archives/C069RUP71/p1643044498033400

Test Plan: Existing tests, and tested manually that the use case with the summary table was fixed

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3233

sandbox/grist/usertypes.py

@@ -287,7 +287,7 @@ class Date(Numeric):
     if value in ("", None):
       return None
     elif isinstance(value, datetime.datetime):
-      return moment.dt_to_ts(value)
+      return moment.date_to_ts(value.date())
     elif isinstance(value, datetime.date):
       return moment.date_to_ts(value)
     elif isinstance(value, (float, six.integer_types)):