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.