(Bug 63636): Handle multiple colons in subpage-supporting namespaces

* Added new test which fails html2wt mode because of a leading "./"
  prefix added (to be investigated separately -- failure is unrelated
  to this patch).

Change-Id: I32a97fa506ab7eee84a41cf371c6d5cd4bf2ef64
Merge "(Bug 63636): Handle multiple colons in subpage-supporting namespaces"