(Bug 46470) More robust handling of default sort

* Default sort key can be:
  - a plain-old string (foo)
  - a string with entities ("foo"bar)
  - partially/fully-templated ({{foo}}bar)

* Thus far, default-sort handling was restricted to the plain-old
  string scenario.

* This patch expands that to handle entities as well as templated
  pieces of the sort key.

* No change in parser test results (TODO: add more)

* Tested on the following cases and verified wt2html and wt2wt:
  - {{DEFAULTSORT:foo}}
  - {{DEFAULTSORT:'foo'bar}}
  - {{DEFAULTSORT:{{foo}}bar}}

Change-Id: I45bbb5c01b00cffb07ac69e237aca63a32730f17