Initial pass at new style Posts Controller 

Initial Controller routes and methods:

- [x] get all posts
- [x] get a post
- [x] get revisions for a post
- [x] create a post
- [x] update a post
- [x] delete a post

Test coverage routes and methods:

- [x] get all posts
- [x] get single post
- [x] get revisions for a post
- [x] create a post
- [x] update a post
- [x] delete a post
Not while keeping backwards-compatibility in core. We can file an issue for it if you'd like.

(Note that this, like much of the file, is ported from v1.)