Merge AK category slots

Addresses the problem discussed in #10

Time constraints corresponding to the different AK categories are introduced. They are added to AKs belonging to the category and fulfilled by timeslots that are contained in a default slot of this category. To that end, this PR implements no. 3 in the "merging" strategy of #10, i.e. we check if we would have to merge partially overlapping timeslots and error out if this is the case. This places the burden to design appropriate schedules on the conference organizers.

AKModel/availability/models.py

@@ -151,9 +151,12 @@ class Availability(models.Model):
         if not other.overlaps(self, strict=False):
             raise Exception('Only overlapping Availabilities can be merged.')
 
-        return Availability(
+        avail = Availability(
             start=min(self.start, other.start), end=max(self.end, other.end)
         )
+        if self.event == other.event:
+            avail.event = self.event
+        return avail
 
     def __or__(self, other: 'Availability') -> 'Availability':
         """Performs the merge operation: ``availability1 | availability2``"""
@@ -168,9 +171,12 @@ class Availability(models.Model):
         if not other.overlaps(self, False):
             raise Exception('Only overlapping Availabilities can be intersected.')
 
-        return Availability(
+        avail = Availability(
             start=max(self.start, other.start), end=min(self.end, other.end)
         )
+        if self.event == other.event:
+            avail.event = self.event
+        return avail
 
     def __and__(self, other: 'Availability') -> 'Availability':
         """Performs the intersect operation: ``availability1 &