fixes https://github.com/TryGhost/Team/issues/2625
- Adds an unique option to the mentions API. Enabling this will only
return the latest mention from each source.
- The frontend can fetch the related sources for each page by doing an
extra request to the mentions API.
ref https://github.com/TryGhost/Team/issues/2421
- added the Mentions API endpoint to Admin
- setup initial mention model in the Ember Store to be able to dev with the endpoint
- added basic routing to access the `/mentions` page that is currently behind feature flags
- Setup basic testing with a mirage mock endpoint.
