Issue: Separate Tab Model from Postbit Factory

In VaultWiki 3, the permissions for all the tabs on the article postbit were calculated within the postbit factory. This meant we had to load the entire factory whenever we just wanted to know if a user could, for example, edit a page.

In VaultWiki 4, this has been broken up into at least 3 models/views. 1 tab model that handles permissions for each tab, at least 1 content view that handles the page content separately from any postbit, and at least 1 postbit factory that lays out the pre-assembled page content. This separation allows us to access permissions without loading more than we need, and prepare articles for different kinds of output than the tabbed postbit.