Overlaps_with is wrong when one proposal has a potential time and the other does not

[Jonty]

proposal.overlaps_with is clearly incorrect when one proposal has a potential time and the other does not:

Website/models/cfp.py

Lines 728 to 735 in c417d32

	def overlaps_with(self, other) -> bool:
	if self.potential_start_date:
	return (
	self.potential_end_date > other.potential_start_date
	and other.potential_end_date > self.potential_start_date
	)
	else:
	return self.end_date > other.start_date and other.end_date > self.start_date

This is because we introduced potential times after overlaps_with, and when we did every proposal would have a potential time. These days this is very occasionally no longer true if someone moves something in the schedule tweaker.

The only places this surfaces is in the clashfinder or in the attendee content interface to warn them when they are scheduling something against content in their village.

Arguably attendees shouldn't know about this, but we should pay attention to it in the clashfinder. Not sure what the "correct" fix is. This is in no way critical.

[Jonty]

After taking a quick look the reason the clashfinder is currently broken is entirely because this function is wrong.

    if prop1.overlaps_with(prop2):
       ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/models/cfp.py", line 735, in overlaps_with
    return self.end_date > other.start_date and other.end_date > self.start_date
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: '>' not supported between instances of 'datetime.datetime' and 'NoneType'

Slightly puzzled as to how this can even happen though - the only content visible on the lineup right now is content that has been scheduled and thus should have start and end times. The clashfinder only looks at things that have been favourited, which means they have to be on the schedule. Regardless, it sucks.

[Jonty]

Ah, withdrawn content can have favourites but not be scheduled 🫠