refs #10438, refs #10106
* Renamed existing pages ctrl
* Splitted posts & pages for Admin API v2
* Added pages JSON input schema for Admin API v2
* Removed single author for Content & Admin API v2
- single author is not documented
- single author usage is deprecated in v0.1
- single author usage is removed in API v2
* Splitted posts & postsPublic controller for v2
* Removed requirement to send `status=all` from Admin API v2
* Removed `status` option from pages Content API v2
* Removed `status` options from Users Admin API v2
refs #9866
- invent preview api, but only used internally
- the idea of a preview api is definitiely reaslistic and came up in the past a couple of times
- by that we don't have to differentiate between pages or posts controller
- still support v0.1
- preview controller is not registered for http, only internal handling
