Temporary bans are incorrectly assumed permanent when infractions overlap

[bast0006]

If a user is temporarily banned, and a moderator attempts to permanently ban said user, the following code will never return anything but None.

https://github.com/python-discord/bot/blob/f94505ac975888b259ea2d07b4442d6d908352f8/bot/exts/moderation/infraction/infractions.py#L490

This is because the site api does not return a duration_or_expiry field, it's named expires_at. This prevents us from re-banning a temporary banned user to permanent without unbanning them first or editing the original infraction.

#2234 updated this code to the newer name, but doesn't update returned get_active_infraction to alter the returned dictionary.

https://github.com/python-discord/bot/blob/0921838112120355aed0936923983b545320edef/bot/exts/moderation/infraction/_utils.py#L159 here

This seems like something we should be using dataclasses of a sort for, so that this can be type-checked?