Say hello to KB discussion. [bug 602958]

Adds a `kbforums` app, which is a clone of the `forums` app with a few
tweaks:

* The top-level `Forum` model is replaced by `wiki.models.Document`.
* The permissions have been simplified. There are no per-document forum
  permissions.
* The `Document` does not track its `last_post`.
* `Post.author` has been renamed to `Post.creator` following our ad hoc
  standard.
* The ability to override `Post.created` and `Post.updated`, only used
  in the data migration, is gone.
* Moving threads is gone. This needs more thought, and moving to the
  `forums` app is non-trivial.

apps/kbforums/__init__.py

@@ -0,0 +1,5 @@
+# The number of threads per page.
+THREADS_PER_PAGE = 20
+
+# The number of posts per page.
+POSTS_PER_PAGE = 20

apps/kbforums/feeds.py

@@ -0,0 +1,72 @@
+from django.contrib.syndication.views import Feed
+from django.shortcuts import get_object_or_404
+from django.utils.feedgenerator import Atom1Feed
+from django.utils.html import strip_tags, escape
+
+from tower import ugettext as _
+
+import forums as constants
+from kbforums.models import Thread
+from wiki.models import Document
+
+
+class ThreadsFeed(Feed):
+    feed_type = Atom1Feed
+
+    def get_object(self, request, document_slug):
+        return get_object_or_404(Document,
+                                 slug=document_slug,
+                                 locale=request.locale)
+
+    def title(self, document):
+        return _('Recently updated threads about %s') % document.title
+
+    def link(self, document):
+        return document.get_absolute_url()
+
+    def items(self, document):
+        return document.thread_set.order_by(
+                '-last_post__created')[:constants.THREADS_PER_PAGE]
+
+    def item_title(self, item):
+        return item.title
+
+    def item_author_name(self, item):
+        return item.creator
+
+    def item_pubdate(self, item):
+        return item.created
+
+
+class PostsFeed(Feed):
+    feed_type = Atom1Feed
+
+    def get_object(self, request, document_slug, thread_id):
+        doc = get_object_or_404(Document,
+                                slug=document_slug,
+                                locale=request.locale)
+        return get_object_or_404(Thread, pk=thread_id, document=doc)
+
+    def title(self, thread):
+        return _('Recent posts in %s') % thread.title
+
+    def link(self, thread):
+        return thread.get_absolute_url()
+
+    def description(self, thread):
+        return self.title(thread)
+
+    def items(self, thread):
+        return thread.post_set.order_by('-created')
+
+    def item_title(self, item):
+        return strip_tags(item.content_parsed)[:100]
+
+    def item_description(self, item):
+        return escape(item.content_parsed)
+
+    def item_author_name(self, item):
+        return item.creator
+
+    def item_pubdate(self, item):
+        return item.created