Change Genre.mass_office field to a choices field

[dchiller]

This is currently a CharField, but as a field that should have only a few options ("Mass", "Office", or "Mass/Office") this could be a choices field. In part, this would clean up the inconsistent formatting in the Mass/Office field on https://cantusdatabase.org/genres/:

The existing value of that field will need to be used to populate the new field (perhaps a one-time management command).