Infraction scheduler raises exception when marking infraction as "permanent" during edit

[SebastiaanZ]

When an existing temporary infraction is turned into a permanent infraction using the !infraction edit functionality, we ask the infraction scheduler to schedule a new task with infraction["expires_at"] = None. However, the scheduler expects to be called only for infractions that have a datetime string mapped to the "expires_at" key. This means that the scheduler will try to parse that None value with dateutil.parse.isoparse, which results in an exception:

Traceback (most recent call last):
  File "/bot/bot/cogs/moderation/scheduler.py", line 415, in _scheduled_task
    expiry = dateutil.parser.isoparse(infraction["expires_at"]).replace(tzinfo=None)
  File "/usr/local/lib/python3.7/site-packages/dateutil/parser/isoparser.py", line 37, in func
    return f(self, str_in, *args, **kwargs)
  File "/usr/local/lib/python3.7/site-packages/dateutil/parser/isoparser.py", line 134, in isoparse
    components, pos = self._parse_isodate(dt_str)
  File "/usr/local/lib/python3.7/site-packages/dateutil/parser/isoparser.py", line 208, in _parse_isodate
    return self._parse_isodate_common(dt_str)
  File "/usr/local/lib/python3.7/site-packages/dateutil/parser/isoparser.py", line 213, in _parse_isodate_common
    len_str = len(dt_str)
TypeError: object of type 'NoneType' has no len()

I think the wisest course of action is tackle this at the level of the infraction edit method, not in the scheduler itself. We should just not try to schedule a new task when we've marked it as permanent. That means this part need to get an additional check:

bot/bot/cogs/moderation/management.py

Lines 131 to 134 in 95ae05c

	if 'expires_at' in request_data:
	self.infractions_cog.cancel_task(new_infraction['id'])
	loop = asyncio.get_event_loop()
	self.infractions_cog.schedule_task(loop, new_infraction['id'], new_infraction)

Important: We should still cancel the existing scheduled task (which we currently do, it happens before the exception), so skipping this if block entirely will break the infraction edit in a more severe way: Cancelling the task is essential to guarantee that we don't accidentally "pardon" a now permanent infraction.