Don't send HTML on post save.

fixes #3956
- adjusts post serialiser to not send HTML content
The API just checks that the payload has a root/top-level property that is an array and that the array's first element isn't empty.  So `{ posts: [ { something: '' } ] }` is all it takes to be considered valid.  Whether or not we SHOULD be doing more validation on incoming payloads is something I've been thinking about for a while now...

But even if we do put in more robust validation down the road, I think it makes sense to allow properties to be optional when they're just tossed out on the backend anyway. We're already removing `uuid` from the payload right now.

Anyway, I think either way gets the job done, just something to think about.