(Bug 5323) Disable parsing of external entities in synsuck and the importer

* creates the DW::XML::RSS/DW::XML::Parser modules to replace their
  XML::* equivalents. Works basically the same, just with a tighter
  configuration
Always use LJ::challenge_check_login
Merge branch '_always-use-challenge_check_login' into release-1.20

* _always-use-challenge_check_login:
  Always use LJ::challenge_check_login
Merge branch '_xml-entity-sec-bug5323' into release-1.20

* _xml-entity-sec-bug5323:
  (Bug 5323) Disable parsing of external entities in synsuck and the importer