Tag API: move to primary document format

[ErisDS]

This issue is a result of the API format discussion in #2362, and is part of a larger project to move our API towards the JSON-API format which is documented in the Epic: #2124. This is one step in the journey towards achieving the Tag JSON object format laid out in #2348.

---

The equivalent task for Posts (#2580) has been done with the PR #2596 and should serve as an example of how to do this for Tags.

There is only one API route / method for tags which is browse accessible via /ghost/api/v0.1/tags/. The response from this method looks like:

[{...}, {...}]

The aim of this issue is to change the format of the tag object in all requests and responses so that the tag object is contained in an array with the key tags:

{
    tags: [{...}, {...}]
}

This will involve changing the response format, updating all of the places in which the response is used, updating the tests and adding coverage where it is missing.

This issue is marked beginner as there is only one API function to change, and that function is only used by the client side model for editor-tag-widget.js.

It would also be appreciated if you would add docs to this (#2125) as you go. Thanks 👍