[Task] Standardize on time to be UTC

[PhuongHoang]

Task Description:
We need to convert all time functions to work with UTC timezone instead of local time. This includes database.

Deliverable(s):
All uses of time in the server code should work with UTC.

Additional Context:
We should be okay if the time is not exposed to the consuming clients. However, /api/v1/themes/versions returns created_at which is in UTC. Without storing time in UTC, we need code to convert the timestamps to UTC.

Let's simplify the server code by making it work with UTC by default. Luxon has good support for this.

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