Make page root post customizable via class PageRoot and &view=
Make page root post customizable via class PageRoot and &view=

Also:
- Remove ViewPageTemplate, no longer needed.
- Specify root post in URL, e.g. `&view=template', when POSTing e.g.
  replies or edits. Pass root post id to callbacks in debiki-lift.js so
  they can include it.
- Remove unused debiki.js Settings functions.

debiki/debiki-server-old,debiki/debiki-server-old,debiki/debiki-server-old,debiki/debiki-server-old