Refactor: is this Node visible


Refactor: is this Node visible

The logic to determine whether a Node is displayed or visible is
scattered in both the model and the template.

We should refactor this logic into a function or a method (maybe on the
Namespace class). This change should make it easier to reason if or how
to present a Node given the current User while supporting Namespace
options to control behavior.

List of things that can effect how a Node is presented:

-   is the node disabled? (a disabled node should be set as a
    placeholder)
-   is the node verified? (by default a Namespace will display
    unverified Nodes)
-   is the node approved? (this matters if the Namespace owner wants to
    approve all comments)
-   is the user a moderator / owner of the Namespace?
-   was the node flagged as spam by a moderator / owner of the
    Namespace?
-   was the node flagged as spam by an end user?

By default a Namespace will display a Node regardless of the state of
its verified or approved attributes: A Namespace owner may set any of
the following to True:

-   hide_until_approved
-   hide_unverified

This changes the behavior to hide nodes by default.