* Preview limits the page dropdown to what's selected previously

* Preview limits the page dropdown to what's selected previously
  (as preserving the full list across preview would be tricky). Userdirs
  were still being offered as an option there, remove them.
* Fix a bug where user A created a page concurrently with user B, and
  when B previewed it would redirect B to A's new page, losing B's work.
  Instead, don't redirect and let conflict handling resolve it.