Byline role management

Allow the parties listed in the byline to optionally contain a role, e.g. one person might be a contributor, another the photographer.

Consider making roles optional, via a setting or a code filter. Perhaps wouldn't be necessary, if no role were ever created, then we could choose to not render the UI to set the role

* [ ] Add a taxonomy for byline role (`byline-role`)
* [ ] When setting the byline, the role should be selectable
* [ ] Optional: add ability to set default role on a profile

TBD: should there be any automatic application of the role when outputting the byline? e.g. `By John Smith (Author) and Jane Doe (Photographer)`