Supply a default value for target_audience for the ART+TECH miniconf

* The base model requires a value here
* But we aren't using one; so there's none on the form
* This change supplies a custom enumeration that's specific for this
particular model, which has one value, which is a default value,
which simply says that the field is N/A
* This does mean that when viewing or reviewing the proposal one sees
the Target Audience field, but it will say N/A.

* Testing has shown that this does not affect the other types which
descend from the base Proposal class; they still use the default
enumeration.