Update `datetime` field to correctly store in UTC

[mackeyguenther]

fixes #1400

1. update datetime field (UI component & validation) to serialize/deserialize the value in UTC time, and render the value in the user's local timezone
2. update datetime documentation to make it clear that datetime stores its value in UTC time, and displays it in the user's local timezone in the editing interface

[changeset-bot[bot]]

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Latest commit: 5b3a070a0a2c23c33f089510f41c352b4b508f02

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