Misc. config cache not cleared when the key is changed.

[Ninjaclasher]

Two possible fixes:

1. The signal here is changed to a pre_save so the old instance can be retrieved. The cache should really be cleared based on the old key rather than the new key.
2. Change the key to only be read-only in the admin page. However, this doesn't prevent the key from being changed from e.g. the Python shell, and the cache still won't be cleared properly in this case.