It is possible to re-record over stock recording with the feature code

Reported here:
https://community.freepbx.org/t/system-recording-plays-before-80-call-and-before-connecting-a-park-pickup/78351/2?u=lgaetz

Steps to repro

1. Set up a system recording using a single system recording (not custom) such as in the forum example 'beep'
2. assign a feature code
3. submit and apply config

Now dial the associated feature code and re-record the prompt. You will see that the new recording overwrites the stock sound file in /en and not a custom file in /en/custom.

The GUI already blocks feature code from being assigned to compound recordings, we need to block for non-custom recordings as well.