Began UI for content-negotiation

- "Creation" form
- Ability to fetch selectors
- List selectors (but currently empty views for them)
Rebuilt distribution
Completed content-negotiation UI

- Adds view/edit/delete capabilities.
Rebuilt distribution
Merge branch 'feature/content-negotiation-ui'

Adds admin UI for creating/updating/removing content negotiation selectors.

TODO: fetch content-negotiation selectors for use with populating the drop-downs
in REST/RPC services.