Make unique constraints on break and lessonsubstitution less tight

Together with 18544e06, updating the existing
migration is safe, because

 a) if the migration failed somewhere due to the multiple leaf nodes, it was
    never applied, and changing it is safe;
 b) if it succeeded, it will be renamed to 0008_ later, and thus
    re-applied

CHANGELOG.rst

@@ -14,6 +14,7 @@ Fixed
 
 * Fix migration names and dependencies
 * Migration path was ambigious
+* Unique constraints for breaks and substitutions were too tight
 
 `2.0b0`_ - 2021-05-21
 ---------------------

aleksis/apps/chronos/migrations/0008_unique_constraints.py

@@ -52,11 +52,11 @@ class Migration(migrations.Migration):
         ),
         migrations.AddConstraint(
             model_name='break',
-            constraint=models.UniqueConstraint(fields=('site_id', 'short_name'), name='unique_short_name_per_site_break'),
+            constraint=models.UniqueConstraint(fields=('validity', 'short_name'), name='unique_short_name_per_site_break'),
         ),
         migrations.AddConstraint(
             model_name='lessonsubstitution',
-            constraint=models.UniqueConstraint(fields=('lesson_period', 'week'), name='unique_period_per_week'),
+            constraint=models.UniqueConstraint(fields=('lesson_period', 'week', 'year'), name='unique_period_per_week'),
         ),
         migrations.AddConstraint(
             model_name='room',

aleksis/apps/chronos/models.py

@@ -451,7 +451,7 @@ class LessonSubstitution(ExtensibleModel, TeacherPropertiesMixin, WeekRelatedMix
             ),
             # Heads up: Link to period implies uniqueness per site
             models.UniqueConstraint(
-                fields=["lesson_period", "week"], name="unique_period_per_week"
+                fields=["lesson_period", "week", "year"], name="unique_period_per_week"
             ),
         ]
         indexes = [
@@ -866,7 +866,7 @@ class Break(ValidityRangeRelatedExtensibleModel):
         verbose_name_plural = _("Breaks")
         constraints = [
             models.UniqueConstraint(
-                fields=["site_id", "short_name"], name="unique_short_name_per_site_break"
+                fields=["validity", "short_name"], name="unique_short_name_per_site_break"
             ),
         ]