Fix up title/slug handling on posts.

Refs #4323
- Remove any titleScratch observers that may be hanging
  around on the post-settings-menu controller from previous posts.
- Change logic around when to regenerate slugs on posts that are
  "(Untitled)" so that slug generation continues after slug no
  longer resembles "untitled."
Merge pull request #4337 from jaswilli/title

Fix up title/slug handling on posts.