Fixed #24513 -- Made sure a model is only rendered once during reloads

This also prevents state modifications from corrupting previous states.
Previously, when a model defining a relation was unregistered first,
clearing the cache would cause its related models' _meta to be cleared
and would result in the old models losing track of their relations.

docs/releases/1.8.1.txt

@@ -25,3 +25,6 @@ Bugfixes
 * Stripped microseconds from ``datetime`` values when using an older version of
   the MySQLdb DB API driver as it does not support fractional seconds
   (:ticket:`24584`).
+
+* Fixed a migration crash when altering
+  :class:`~django.db.models.ManyToManyField`\s (:ticket:`24513`).