website: missing timezone conversion back to UTC on save

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 (and the saved value would be the exact value written by the user, rather than the corresponding UTC value)

Fix that by applying `context_timestamp`'s conversion backwards, from the context/user's timezone back to UTC, before saving the field's value.