Reuse page preview parses by using the edit stash system

* This also changes previews to render section edit tokens but
  remove them on output, avoiding cache fragmentation.
* Also shortened the resulting getStashKey() value.

Change-Id: Ic8fa87669106b960c76912b864788b781f6ee2e6
(cherry picked from commit 17431af1544ca4d5869656296576b3018b57d8f6)