[FIX] website: missing timezone conversion back to UTC

In t-field, datetime fields (formatted and not formatted versions) are
converted to the context/user's timezone (through
fields.datetime.context_timestamp) when displayed, but were saved without
converting back so the next display would go forward (or back) of the user's
tzoffset.

Fix that by applying context_timestamp's conversion backwards, from the
context/user's timezone back to UTC, before saving the field's value.