Use format_infaction datetime util in talentpool

The watchchannel ABC defined its own private utility function to
format ISO datetime strings to something more human-readable. I have
removed this private utility function and replaced the calls to it
with calls to the new `format_infraction` utility function defined in
bot.utils.time.

In addition, I've changed the utility function to use `dateutil` to
parse the datetime string, since `dateutil.parser.isoparse` supports
the strings our API generates out of the box. With the built-in
`datetime.datetime.fromisoformat`, we needed to prepare the string by
slicing of the `Z` timezone indicator.

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