REST support for posts and pages

Adds REST implementations for posts and pages:

- All post and page management is now done through `PostService`
- Categories are now automatically created if included in the post response
- There are a few workarounds for API bugs that are pending resolution

Well, this took a while :smile: 

I'll be testing this more over the next days, but since it's a big PR you can start reviewing the code and testing as well.

Fixes #1901 
I wasn't sure here. There is remote-specific code in the service, but at the same time, the options code is specific to infinite scrolling and needs to look at core data to calculate the options.
I think the remote classes shouldn't know about core data objects although so far we're passing `blog` objects to pretty much every method. I'll probably fix that when adding full REST support to `BlogService` and maybe adding a `RemoteBlog` object.