Enforce uniqueness of EventMetaProperty names in DB (Z#23231419) (#6106)

After enforcing this on application level in commit 4c37351, we can now also add
a unique constraint on DB level and a migration to fix any left over non-unique entries.

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Co-authored-by: Raphael Michel <mail@raphaelmichel.de>

src/pretix/base/models/event.py

@@ -1851,6 +1851,7 @@ class EventMetaProperty(LoggedModel):
 
     class Meta:
         ordering = ("position", "name",)
+        unique_together = ('organizer', 'name')
 
     @property
     def choice_keys(self):