* Changed internal version number to 6.21

* Changed internal version number to 6.21
* Added missing VIEW_* defines in citadel.h (they're used only by WebCit
  for now, which is why Citadel didn't need them, but they should be there
  anyway for completeness)
* Removed the last vestiges of the built-in spam filter.  It had been
  commented out, but it's definitely never coming back now, so away it goes.
* Updated some of the docs