Fixes to expansion of invalid template tokens

* {{<invalid-tpl-target>|{{echo|foo}}|b}} on expansion would
  lose all nested template encapsulations because the template
  tokens returned by the template handler were not subject to
  additional template expansion.

* In this patch, the exploded token is processed in a separate
  content pipeline that fixes the above problem.

* In addition, this patch also gets rid of the template-wrapper
  around the invalid template token. This leads to a bunch of
  parser test failures because our nowiki escaping code in the
  serializer unconditionally nowikis {{ and }} tokens.

Change-Id: Ied3b465a27b1aab43192c1e7ae9c876ab62c051b
Merge "Fixes to expansion of invalid template tokens"