Fix serializing/deserializing Tags on save from the Editor

closes #2947
- make a PostSerializer to embed the tags objects in the posts POST/PUT request
- on editor save, clear out tags from the post and data store that have `id: null`

There's no easy or obvious way in Ember Data to update the client-generated tags once they return from the server with the id. I tried for hours with a Tag serializer before giving up. Instead, on return from the server, I allow the saved tags to attach themselves to the post, and delete the otherwise identical client-generated tags. The Editor PR will also delete unsaved client-generated tasks on transition away from the editor.