Auto-save changes

Refs #4259
- Auto-save new post when title loses focus.
- If a post has '(Untitled)' for a title, regenerate slugs
  on all title changes--same behavior as a new post that does
  not yet have a slug.
- Adjust some functional tests to handle the automatic transition
  from editor/new to editor/editor and the URL changes that go
  along with it.