Display `InheritedValue` for `TopicReference` attribute type

[JeremyCaney]

Previously, we implemented InheritedValue support (alongside DefaultValue and ImplicitValue) for most attribute types. This was not implemented into TopicReference, however, as this introduces specific challenges that aren't trivial to mitigate. This will require further research and is thus being deferred for further evaluation.

This will likely require additional JavaScript to be introduced to display a “phantom” token with the InheritedValue—but it should only be displayed if there isn’t an existing value. It may be that the tokenized input plugin we're using supports something like this, though it's unlikely.