Add support for default account code

Using FreePBX going through A2billing we used to come across problems with call forwarding and other situations where the call would not have an account code associated with it. So we worked to set an account code for these calls in the dialplan, but now we would prefer to have it in the GUI for editing by non-technical administrators.

This patch allows you to force an account code to apply to all calls, or just to calls without a pre-existing account code.