Fix Inconsistent use of UTC time in DateTime class

[BrendanSchell]

Description

Main change is to update the DateTime class to treat timezone consistently as UTC to avoid some inconsistencies between the use of UTC and local time.

Other minor updates:

• replace use of utctimestamp, utcnow functions in DateTime, BaseDateTime classes
• update sub op of DateTime / BaseDateTime to return timedelta instead of DateTime (not sure if it made sense before or was even being used)
• fixed Asset repr when either private or mock data was empty action object (noticed incidentally trying to replicate bug)

Closes https://github.com/OpenMined/Heartbeat/issues/1475.

Affected Dependencies

List any dependencies that are required for this change.

How has this been tested?

• Tested in notebook to make sure times / delta were correct in syncing widget.

Checklist

• [] I have followed the Contribution Guidelines and Code of Conduct
• [] I have commented my code following the OpenMined Styleguide
• [] I have labeled this PR with the relevant Type labels
• My changes are covered by tests

[snwagh]

Awesome work, LGTM!