Implement 409 Conflict where AETitle exists - POST /config/ae

**Is your feature request related to a problem? Please describe.**
For Aide it would be useful for the endpoint POST /config/ae to return a 409 Conflict when it already exists. If this is not the case we wouldn't have a way to check if it exists aside from making another call.


**Describe the solution you'd like**
When AETitle exists, a 409 is returned.
