Add Operetta as a genre

[alliyya]

Context:
The Genre vocab has operetta as an alternative label for opera.

Operetta is a different musical genre, somewhere between an opera and a musical
see https://www.operaforall.co.uk/opera-vs-operetta/

We had previously said we'd add a new term here and the item is checked off but the term is not in the Ontology/Vocabulary and is instead an altLabel under Opera.

Task:

• Remove altLabel from opera
• Create term operetta
• Do the same for the Genre Vocabulary

Let's list them under MusicalGenre and OperaGenre. @SusanBrown Does that sound right to you?

I think this is a relatively high priority since the Genre Vocabulary is being used for the Taxonomy generation for CWRC, but I'll double-check with Mihaela.