BranchFeedBase needs cleanup

[Tim Penhey]

The BranchFeedBase should be refactored to use the BranchCollection properly, instead of using

    def getPublicRawItems(self):
        """Private branchess are not to be shown in feeds.

        The list of branches is screened to ensure no private branches are
        returned.
        """
        return [branch
                for branch in self._getRawItems()
                if not branch.private]

it should use the BranchCollection.visibleByUser(None).