refs https://github.com/TryGhost/Team/issues/2503
This is in the MentionController atm as it's considered a presentation
concern. We might want to consider moving this into the MentionsAPI in
future so that we can simplify the controller and even remove it
completely in favour of putting the data-mapping in the endpoint definition.
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.
