Fixes infraction display duration off-by-one discrepancy

[ionite34]

Closes #2217, Closes #2130

Overview

Fixes the issue of displayed infractions being off from the originally requested duration.

• Implements new infraction duration calculations using a last_applied time field for infractions.
• Adds refactors for the last_applied model attribute in the merged PR https://github.com/python-discord/site/pull/751

Test Example

• Infraction messages now display the correct intended applied time and are no longer off by one or more units.

• DMs and most areas where durations are used have also been updated

Related Fixes and improvements

• The ban command help message did not accurately reflect its ability to accept expiry ISO times

• This has been improved as such

Design and API Notes

• Commands for durations have been modified from Expiry to use a new alias DurationOrExpiry

Expiry = t.Union[Duration, ISODateTime]
->
DurationOrExpiry = t.Union[DurationDelta, ISODateTime]

The added type, in converters.py, is a union between the types dateutils.timedelta and datetime.datetime respectively.

• The post_infractions method of _utils.py has an updated parameter name and type. Associated usages have also been updated.
• The updated name is to reflect that this new parameter may now accept either a timedelta or datetime marking the absolute expiry date. Originally the expires_at parameter only accepted a datetime marking the expiry date.

async def post_infraction(
		...
-     expires_at: datetime = None,
+     duration_or_expiry: t.Optional[DurationOrExpiry] = None,
		...
) -> t.Optional[dict]: