Round-trip relative and pipe-tricky links

* Add a title and ns member to env.page

* Use that to check whether a page is in a namespace that has subpages
  enabled

* Add a test for links of the form [[../../foo]] and [[../../foo/]]

* Round-trip relative links like this

Change-Id: I04acb2e87fd6ae7354da451a7d36a3bc65e876c8
Merge "Round-trip relative and pipe-tricky links"