migrations: Mark RunPython statements elidable.

This will make django automatically remove them when we run
squashmigrations. There are still some RunSQL statements which
we will have to take care of manually.

zerver/migrations/0242_fix_bot_email_property.py

@@ -20,5 +20,6 @@ class Migration(migrations.Migration):
 
     operations = [
         migrations.RunPython(fix_bot_email_property,
-                             reverse_code=migrations.RunPython.noop),
+                             reverse_code=migrations.RunPython.noop,
+                             elidable=True),
     ]