Add Model.isOwner

This is to replace the isOwner method duplicated in authsome modes:

• https://gitlab.coko.foundation/yannisbarlas/authsome-editoria/blob/master/src/index.js (currently broken as it expects Model.owners to be an array of objects)
• https://gitlab.coko.foundation/pubsweet/authsome/blob/master/src/modes/blog/index.js

Both of those check that object is set, but it seems unlikely that isOwner would be called when the object isn't set?