Tag API: Primary Document Format

Closes #2605

- Change tags browse() response to { tags: [...] }
- Update client side collection to use nested tags document
- Update test references to use response.tags

Added some basic [Tags API Docs](https://github.com/TryGhost/Ghost/wiki/%5BWIP%5D-API-Documentation#tag) as well.

#easywin #beginner4lyfe